Handling update exceptions

When searching for updates one of the update site doesn't return response it causes whole JUpdater to crash: `0 - No Http response received`.

JUpdater has been switched from @fopen to JHttp, which throws exceptions.

Issue tracker: [#29973](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=29973)