[2.5x] Import the user plugins for onUserLoginFailure

Backport https://github.com/joomla/joomla-cms/pull/2761 to Joomla 2.5

The onUserLoginFailure trigger is a user plugin method however on the failure code path, it isn't loaded. So this changes that to put it on the main code path ahead of the failure check.

### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=34070&start=0