Feature request

Feature description

With #924 merged in, a user is able to provide a callback for when an participant on the topic is discovered but unable to be matched due to incompatible QoS policies. This feature request is for leveraging this feature even when a user does not provide a custom callback function, by setting a default callback function that prints a warning (initially suggested at ros2/ros2#822 (comment)) on the discovery of an incompatible participant on the topic.

Implementation considerations

What should the behavior be if the underlying middleware does not support this kind of QoS event callback?