Fixed #15287 -- Added translation markers to user-facing error messages in...

Fixed #15287 -- Added translation markers to user-facing error messages in class-based generic views. Thanks to szczav for the report and draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15533 bcc190cf-cafb-0310-a4f2-bffc1f526a37