[1.1.X] Fixed #12554: Silence exceptions that have specified...

[1.1.X] Fixed #12554: Silence exceptions that have specified silent_variable_failure=True. Thanks Thomas Steinacher, copelco, mlavin. 

r12823 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12824 bcc190cf-cafb-0310-a4f2-bffc1f526a37