Fixed #9031 -- Clarified the error message when the label used to run a...

Fixed #9031 -- Clarified the error message when the label used to run a specific test case method doesn't name a valid test case class. Thanks to Thomas Guettler for the suggestion and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9879 bcc190cf-cafb-0310-a4f2-bffc1f526a37