Preserve error types during translation

By preserving the exception type, more fine-grained error handling can be performed via client-side logic (e.g. redirect to a search page if a PDF is not found, or to a ticket system in case of invalid PDF files).

The original exception is now re-thrown.

Fixes mozilla#11658

web/app.js

@@ -817,7 +817,7 @@ const PDFViewerApplication = {
 
         return loadingErrorMessage.then(msg => {
           this.error(msg, { message });
-          throw new Error(msg);
+          throw exception;
         });
       }
     );