Corrected a problem with the error message when the nominated database backend...

Corrected a problem with the error message when the nominated database backend doesn't exist. Thanks to Ramiro Morales for the report and fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8297 bcc190cf-cafb-0310-a4f2-bffc1f526a37