Filtered out 'base' from database backend choices error message.

tests/backends/test_utils.py

@@ -0,0 +1,17 @@
+from django.core.exceptions import ImproperlyConfigured
+from django.db.utils import load_backend
+from django.test import SimpleTestCase
+from django.test.utils import str_prefix
+from django.utils import six
+
+
+class TestLoadBackend(SimpleTestCase):
+    def test_load_backend_invalid_name(self):
+        msg = str_prefix(
+            "'foo' isn't an available database backend.\n"
+            "Try using 'django.db.backends.XXX', where XXX is one of:\n"
+            "    %(_)s'mysql', %(_)s'oracle', %(_)s'postgresql_psycopg2', %(_)s'sqlite3'\n"
+            "Error was: No module named %%s"
+        ) % "foo.base" if six.PY2 else "'foo'"
+        with self.assertRaisesMessage(ImproperlyConfigured, msg):
+            load_backend('foo')