[1.1.X] Fixed #11191 - Admin throws 500 instead of 404 for PK of incorrect type

  
Thanks to mmachine for report and test, and Chris Beaven for the patch

Backport of r12011 from trunk



git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12012 bcc190cf-cafb-0310-a4f2-bffc1f526a37