fix for: #33917

As discussed on the issue tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33917&start=0

When a user attempts to access a URL they are not authorized to it is not sufficient to just show and error. Execution must be terminated as well. The fix is to throw an exception in a similar fashion to Joomla!2.x