Fix some issues with creating a new thread

There was an error discovered on translatewiki.net when LQT started
using the mediawiki.api system instead of $.post. This commit:
1) Changes around some code to fix the (unused with mediawiki.api) non-AJAX
   fallback via error callbacks
2) Fixes the original error

See I006ffbe7 for more information.

Change-Id: I764b37266756508e5a2811fe4fabe327207118d7