fix(sandbox): better error reporting broken processor file

lib/process/child-pool.js

@@ -64,7 +64,7 @@ ChildPool.prototype.retain = function(processFile) {
       })
       .catch(err => {
         this.remove(child);
-        throw new Error(err);
+        throw err;
       });
   });
 };
@@ -115,8 +115,8 @@ async function initChild(child, processFile) {
     const onMessageHandler = msg => {
       if (msg.cmd === 'init-complete') {
         resolve();
-      } else {
-        reject(msg.value);
+      } else if (msg.cmd === 'error') {
+        reject(msg.error);
       }
       child.off('message', onMessageHandler);
     };

lib/process/master.js

@@ -50,9 +50,10 @@ process.on('message', msg => {
         processor = require(msg.value);
       } catch (err) {
         status = 'Errored';
+        err.message = `Error loading process file ${msg.value}. ${err.message}`;
         return process.send({
           cmd: 'error',
-          value: `Error loading process file ${msg.value}. ${err.toString()}`
+          error: err
         });
       }