[1.2.X] Fixed #3055 -- Validate that models target of a GenericRelation have a...

[1.2.X] Fixed #3055 -- Validate that models target of a GenericRelation have a GenericForeignKey field.

Thanks jason for diagnosing the problem and Marcos Moyano for the patch.

Backport of [14563] from trunk

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14565 bcc190cf-cafb-0310-a4f2-bffc1f526a37