refactor: make client- and operator-facing error messages clearer and actionable

test/integration/features/chromium_convert_html.feature

@@ -735,7 +735,7 @@ Feature: /forms/chromium/convert/html
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF split mode, while others may have failed to split due to different issues
+      The requested split mode is not supported, or no PDF engine could process it. Valid modes: 'intervals', 'pages'.
       """
     When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/html" endpoint with the following form data and header(s):
       | files | testdata/page-1-html/index.html | file  |
@@ -744,7 +744,7 @@ Feature: /forms/chromium/convert/html
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF format, while others may have failed to convert due to different issues
+      The requested PDF format is not supported, or no PDF engine could apply it. Valid formats include PDF/A-1b, PDF/A-2b, PDF/A-3b, and PDF/UA.
       """
     When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/html" endpoint with the following form data and header(s):
       | files | testdata/page-1-html/index.html | file  |

test/integration/features/chromium_convert_markdown.feature

@@ -652,7 +652,7 @@ Feature: /forms/chromium/convert/markdown
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF split mode, while others may have failed to split due to different issues
+      The requested split mode is not supported, or no PDF engine could process it. Valid modes: 'intervals', 'pages'.
       """
     When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/markdown" endpoint with the following form data and header(s):
       | files | testdata/page-1-markdown/index.html | file  |
@@ -662,7 +662,7 @@ Feature: /forms/chromium/convert/markdown
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF format, while others may have failed to convert due to different issues
+      The requested PDF format is not supported, or no PDF engine could apply it. Valid formats include PDF/A-1b, PDF/A-2b, PDF/A-3b, and PDF/UA.
       """
     When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/markdown" endpoint with the following form data and header(s):
       | files | testdata/page-1-markdown/index.html | file  |

test/integration/features/chromium_convert_url.feature

@@ -815,7 +815,7 @@ Feature: /forms/chromium/convert/url
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF split mode, while others may have failed to split due to different issues
+      The requested split mode is not supported, or no PDF engine could process it. Valid modes: 'intervals', 'pages'.
       """
     Given I have a static server
     When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/url" endpoint with the following form data and header(s):
@@ -825,7 +825,7 @@ Feature: /forms/chromium/convert/url
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF format, while others may have failed to convert due to different issues
+      The requested PDF format is not supported, or no PDF engine could apply it. Valid formats include PDF/A-1b, PDF/A-2b, PDF/A-3b, and PDF/UA.
       """
     Given I have a static server
     When I make a "POST" request to Gotenberg at the "/forms/chromium/convert/url" endpoint with the following form data and header(s):

test/integration/features/libreoffice_convert.feature

@@ -313,7 +313,7 @@ Feature: /forms/libreoffice/convert
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF split mode, while others may have failed to split due to different issues
+      The requested split mode is not supported, or no PDF engine could process it. Valid modes: 'intervals', 'pages'.
       """
     When I make a "POST" request to Gotenberg at the "/forms/libreoffice/convert" endpoint with the following form data and header(s):
       | files | testdata/pages_3.docx | file  |
@@ -322,7 +322,7 @@ Feature: /forms/libreoffice/convert
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      A PDF format in '{PdfA:foo PdfUa:false}' is not supported
+      The PDF format 'foo' is not supported. Valid formats include PDF/A-1b, PDF/A-2b, PDF/A-3b, and PDF/UA.
       """
     When I make a "POST" request to Gotenberg at the "/forms/libreoffice/convert" endpoint with the following form data and header(s):
       | files | testdata/page_1.docx | file  |

test/integration/features/pdfengines_convert.feature

@@ -93,7 +93,7 @@ Feature: /forms/pdfengines/convert
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF format, while others may have failed to convert due to different issues
+      The requested PDF format is not supported, or no PDF engine could apply it. Valid formats include PDF/A-1b, PDF/A-2b, PDF/A-3b, and PDF/UA.
       """
     When I make a "POST" request to Gotenberg at the "/forms/pdfengines/convert" endpoint with the following form data and header(s):
       | files | testdata/page_1.pdf | file  |

test/integration/features/pdfengines_merge.feature

@@ -108,7 +108,7 @@ Feature: /forms/pdfengines/merge
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF format, while others may have failed to convert due to different issues
+      The requested PDF format is not supported, or no PDF engine could apply it. Valid formats include PDF/A-1b, PDF/A-2b, PDF/A-3b, and PDF/UA.
       """
     When I make a "POST" request to Gotenberg at the "/forms/pdfengines/merge" endpoint with the following form data and header(s):
       | files | testdata/page_1.pdf | file  |

test/integration/features/pdfengines_metadata.feature

@@ -120,7 +120,7 @@ Feature: /forms/pdfengines/metadata/{write|read}
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should contain string:
       """
-      At least one PDF engine cannot process the requested metadata
+      The requested metadata could not be written
       """
 
   Scenario: POST /forms/pdfengines/metadata/write (Reject Group-Prefixed Dangerous Tag)

test/integration/features/pdfengines_split.feature

@@ -283,7 +283,7 @@ Feature: /forms/pdfengines/split
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF split mode, while others may have failed to split due to different issues
+      The requested split mode is not supported, or no PDF engine could process it. Valid modes: 'intervals', 'pages'.
       """
     When I make a "POST" request to Gotenberg at the "/forms/pdfengines/split" endpoint with the following form data and header(s):
       | files     | testdata/pages_3.pdf | file  |
@@ -294,7 +294,7 @@ Feature: /forms/pdfengines/split
     Then the response header "Content-Type" should be "text/plain; charset=UTF-8"
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested PDF format, while others may have failed to convert due to different issues
+      The requested PDF format is not supported, or no PDF engine could apply it. Valid formats include PDF/A-1b, PDF/A-2b, PDF/A-3b, and PDF/UA.
       """
     When I make a "POST" request to Gotenberg at the "/forms/pdfengines/split" endpoint with the following form data and header(s):
       | files     | testdata/pages_3.pdf | file  |

test/integration/features/pdfengines_stamp.feature

@@ -83,7 +83,7 @@ Feature: /forms/pdfengines/stamp
     Then the response status code should be 400
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested stamp source type, while others may have failed due to different issues
+      The requested stamp source is not supported, or no PDF engine could process it. Valid sources: 'text', 'image', 'pdf'.
       """
 
   @download-from

test/integration/features/pdfengines_watermark.feature

@@ -83,7 +83,7 @@ Feature: /forms/pdfengines/watermark
     Then the response status code should be 400
     Then the response body should match string:
       """
-      At least one PDF engine cannot process the requested stamp source type, while others may have failed due to different issues
+      The requested stamp source is not supported, or no PDF engine could process it. Valid sources: 'text', 'image', 'pdf'.
       """
 
   @download-from