I'd like to preserve this as a debug message. Canceling goals is something that can happen a lot, and if you have multiple controllers running, the log traffic can be a bit overwhelming. I use this statement only when debugging, to get confirmation that the cancel callback was triggered. Also, note that when trajectory execution is stopped because an [empty message was received](https://github.com/ros-controls/ros_controllers/blob/hydro-devel/joint_trajectory_controller/include/joint_trajectory_controller/joint_trajectory_controller_impl.h#L459), we also issue a log statement with debug severity to prevent spamming the logger too much.

I'm OK with the change in the string message though.