Mention shared_db_wrapper when failing

Author: ktosiek

pytest_django/fixtures.py

@@ -223,7 +223,9 @@ def wrapper(request):
 
         with _django_cursor_wrapper:
             if not connection.features.supports_transactions:
-                raise Exception("Database doesn't support transactions.")
+                raise Exception(
+                    "shared_db_wrapper cannot be used when "
+                    "the database doesn't support transactions.")
 
         exc_type, exc_value, traceback = DummyException, DummyException(), None
         # Use atomic instead of calling .savepoint* directly.