Jonas Jenwald 028d7c0950 Ensure that PDFViewerApplication.error outputs proper messages in FIREFOX/MOZCENTRAL builds
*It appears that this accidentally broke with PR 8394.*

Currently, the following will be printed in the console:
```
An error occurred while loading the PDF.
[object Promise],[object Promise]
```

With this patch we'll again get proper output, e.g. something with this format:
```
An error occurred while loading the PDF.
PDF.js v? (build: ?)
Message: unknown encryption method
```
2017-08-28 13:49:40 +02:00
..