Fixed #12574 -- Removed an unnecessary exception catch from the system runtest...

Fixed #12574 -- Removed an unnecessary exception catch from the system runtest script, which could hide failing tests. Thanks to CarlFK for the report, and Ramiro Morales for the polish.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13616 bcc190cf-cafb-0310-a4f2-bffc1f526a37