Bug 1062806 - Use the new shared helper for loading JSON files in the co..

...st control app

https://bugzilla.mozilla.org/show_bug.cgi?id=1062806
Notice the `else` is incorrect. The original function does not handle the `onerror` case but for the `onload` case, even if the JSON was failed to load we provided a default object and **continue**. To keep the semantics, remove the `else` clause.