Add exception_callback to ActionServer

[tonynajjar]

Feature request

Feature description

This PR added the handling of exceptions in the user's execute_callback. I would like to execute some commands whenever an action server has an exception, for example publishing an error message to a topic. I therefore propose to add exception_callback as a new argument to ActionServer, which is a user-provided callback executed when an action server has an exception.

Implementation considerations

[fujitatomoya]

I agree with this approach.

This would be worth to discuss in general for ROS 2 client libraries perspective, including rclcpp.

• There are other classes that exception_callback should be added as well?
• If exception_callback is not added by user application, raise the exception up to user application?

I think it would be nice to discuss on basic policy so that we can keep the behavior consistent.

@ros2/team