Fixed #12594 -- Ensured that a meaningful exception is raised when the...

Fixed #12594 -- Ensured that a meaningful exception is raised when the urlconf_module is None. Thanks to buriy for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12854 bcc190cf-cafb-0310-a4f2-bffc1f526a37