Fix erp5_web_service in Python 2
The if in this change was not being reached in Python 2, because in this version the repr includes a comma after the message. I have chosen instead to do the same that was being done before commit 9f4cf295, but changing message (which does not exist in Python 3) to args[0] which can be used in both Python versions.
The tests pass locally and a test suite has been launched.