Fix error on com_joomlaupdate if update.joomla.org is not available.

### Tracker

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33832&start=0

### How to test

1. install Joomla 3.3.0
2. disable network (or use it without internet connection)
3. access the Joomla Updater
4. it send a error that he can't access update.joomla.org
5. apply patch
6. test again (possible clear the updater cache)
7. The "normal" Message by joomla (can not open the URL) apperars and the URL will "cached". (same behavior as in 2.5)