Work around localhost exception issues in add_user when connected to MongoDB >= 2.7.1

Starting with MongoDB 2.7.1 the scope of the localhost exception for authentication was dramatically narrowed. It is no longer possible to call the userInfo command unauthenticated, even when the localhost exception is in effect. A workaround for this issue is to catch the exception and call createUser if the error code is 13 (Unauthorized). Any exception from the createUser call should propagate to the user application.