Bug 1062803 - Replace the custom functions used to load JSON files with ...

...the new shared helper in the system app

https://bugzilla.mozilla.org/show_bug.cgi?id=1062803
This is not right, the old code invoked ``resolve()`` with ``null`` in case of failure so either you do that in your ``reject`` handler (which is weird but consistent with the existing code) or you modify all the code which used to call _loadJSON() to comply with this. Anyway I wouldn't create a new error in the ``reject`` handler since the original code didn't do it. Rely on the error returned by the helper.