Stop API error dialog popping up if you navigate away from the page while Parsoid loads

By checking response.status per this page:
http://stackoverflow.com/questions/9229005/how-to-handle-jquery-ajax-post-error-when-navigating-away-from-a-page

Bug: 51546
Change-Id: I80830b4af1f950369b73497c767cdee4806e7ce6
Merge "Stop API error dialog popping up if you navigate away from the page while Parsoid loads"