Fix Javascript error handling message during update

![joomlaupdatejserror](https://cloud.githubusercontent.com/assets/2002921/3539067/604c603c-0837-11e4-8d69-fbb03f33e59a.png)

Joomla issue tracker URL,
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33935