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
Just see `null` is a valid JSON value. I mean, this JSON document:
```javascript
null
```
Is valid JSON and must be parsed as the value `null`.