tf2_buffer_server aborts goal without setting error if timeout is set in goal

Galactic from Packages

I wrote an action client for the tf2_buffer_server and tested it with invalid frames while setting the timeout in the goal. I did not check the goal-status, but only the error flag in the result. This works if I don't set the timeout, because the result.error.error is set to LOOKUP_ERROR https://github.com/ros2/geometry2/blob/74b873a497f405f4296bb1c5de4023ff45eb9007/tf2_ros/src/buffer_server.cpp#L75

If however I set the timeout, the action is aborted without setting the error:
https://github.com/ros2/geometry2/blob/74b873a497f405f4296bb1c5de4023ff45eb9007/tf2_ros/src/buffer_server.cpp#L98

I know that you should check the goal-status, but I think it would be nicer to have a consistent result so that we always have the error filled if the action did not succeed.

My proposal would be to simply set
result->error.error = result->error.TIMEOUT_ERROR;
if the timeout was triggered.