CMUpdater: use different notification icon when download fails

Use case:
1. User starts downloading update and puts phone away
2. After some time, user looks at the phone and sees CMUpdater icon.
3. User pulls the notification shade expecting the update ready to be installed.
4. CRAP, IT FAILED!

This patch changes "failed" notification icon to a default warning icon, so user
will know that something went wrong. Prevents frustration from incomplete updates.

Change-Id: I6896497eccb9539d649291310b68b6acb7169219