Fixed #15765 -- Stopped showing an odd error message when using the...

Fixed #15765 -- Stopped showing an odd error message when using the staticfiles enabled runserver management command and trying to serve files from STATIC_URL at the same time.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16503 bcc190cf-cafb-0310-a4f2-bffc1f526a37