[1.1.X] Fixed #12689: Fixed admin validation to report an error on invalid...

[1.1.X] Fixed #12689: Fixed admin validation to report an error on invalid exclude specification. Thanks for report to bparker and for patch with tests to ramiro. 

r12734 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12735 bcc190cf-cafb-0310-a4f2-bffc1f526a37