Standard traits exception handler `_log_exception` should not blindly create handler

I'm referring to the method `_log_exception` of the class `traits.trait_notifiers.NotificationExceptionHandler`, which will add `logging.StreamHandler` to the list of the handlers managed by the `logging` library. This makes it difficult for users of `traits` to keep stdout/stderr free from exception messages. Instead, it should either just not add a logging handler, or at least check if any other handler is registered.