Fixed #22279 -- Prevented dummy backend going through DatabaseErrorWrapper

Thanks Daniel Hahler for the report and Tim Graham for the review.

django/db/backends/dummy/base.py

@@ -60,6 +60,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
     # do something raises complain; anything that tries
     # to rollback or undo something raises ignore.
     _cursor = complain
+    ensure_connection = complain
     _commit = complain
     _rollback = ignore
     _close = ignore

tests/backends/tests.py

@@ -41,6 +41,8 @@ class DummyBackendTest(TestCase):
         conns = ConnectionHandler(DATABASES)
         self.assertEqual(conns[DEFAULT_DB_ALIAS].settings_dict['ENGINE'],
             'django.db.backends.dummy')
+        with self.assertRaises(ImproperlyConfigured):
+            conns[DEFAULT_DB_ALIAS].ensure_connection()
 
 
 @unittest.skipUnless(connection.vendor == 'oracle', "Test only for Oracle")