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
After thinking a little about this I think that we should really check the response field instead. The status depends on the protocol used and doesn't represent failure completely though it can be used to provide information in case of certain failures.

Case in point, an empty or invalid JSON file which is present on the server will yield a ``status`` of 200 yet the ``response`` field will be null. So instead of checking for the status let's check if ``response`` is not null.