Bug 1069808 - Make the LazyLoader.getJSON() helper invoke reject() in ca...

...se of errors instead of resolving the promise to null

https://bugzilla.mozilla.org/show_bug.cgi?id=1069808
Let's drop the ERROR part of the string (since this is already an error object) and use the status only for informative purposes, e.g.:
```javascript
reject(new Error('No valid JSON object was found (' + xhr.status + ' ' + xhr.statusText + ')'));
```