Fixed #8238 -- If an invalid database backend is mentioned in settings and the

environment doesn't support os.listdir(), we crashed whilst trying to construct
the friendly error message. This was not so friendly.

This patch handles that case (which occurs in real life in Google App Engine).
Patch from Guido van Rossum.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8424 bcc190cf-cafb-0310-a4f2-bffc1f526a37