Fixed #3505 -- Added handling for the error raised when the user forgets the comma in a single element tuple when defining AUTHENTICATION_BACKENDS. Thanks for the help identifying this problem, Mario Gonzalez <gonzalemario@gmail.com>.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@5678 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/auth/__init__.py

@@ -12,6 +12,8 @@ def load_backend(path):
         mod = __import__(module, {}, {}, [attr])
     except ImportError, e:
         raise ImproperlyConfigured, 'Error importing authentication backend %s: "%s"' % (module, e)
+    except ValueError, e:
+        raise ImproperlyConfigured, 'Error importing authentication backends. Is AUTHENTICATION_BACKENDS a correctly defined list or tuple?'
     try:
         cls = getattr(mod, attr)
     except AttributeError: