Fixed #17047 -- Improved django.db.utils.load_backend to raise a better...

Fixed #17047 -- Improved django.db.utils.load_backend to raise a better exception when using a unqualified database backend name. Thanks, Jonas Obrist.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17274 bcc190cf-cafb-0310-a4f2-bffc1f526a37