[1.2.X] Fixed #12803 - Added styling for 'error' and 'warning' messages in admin.

  
Thanks to bboli and sebastian_noack for the report, DrMeers for the patch.

Backport of [13393] from trunk.  Considered a bug (and therefore backported)
due to error and warning messages being styled with green tick previously.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13394 bcc190cf-cafb-0310-a4f2-bffc1f526a37