[1.2.X] Fixed #12574 -- Removed an unnecessary exception catch from the system...

[1.2.X] 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.

Backport of r13616 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13617 bcc190cf-cafb-0310-a4f2-bffc1f526a37