[ROS 2] Error messages if action server exists

[jacobperron]

If there is an action server on the ROS graph, when rqt_service_caller starts we see errors message about invalid service types.

To reproduce, start turtlesim

ros2 run turtlesim turtlesim_node

And start rqt_service_caller:

ros2 run rqt_service_caller rqt_service_caller 

Example error message:

[ERROR] [1589841268.191775247] [get_service_class]: Malformed srv message_type: turtlesim/action/RotateAbsolute_GetResult
ServiceCaller.on_refresh_services_button_clicked(): could not get class turtlesim/action/RotateAbsolute_GetResult of service /turtle1/rotate_absolute/_action/get_result. Skipping it.
[ERROR] [1589841268.192278390] [get_service_class]: Malformed srv message_type: turtlesim/action/RotateAbsolute_SendGoal
ServiceCaller.on_refresh_services_button_clicked(): could not get class turtlesim/action/RotateAbsolute_SendGoal of service /turtle1/rotate_absolute/_action/send_goal. Skipping it.

I don't necessarily expect the service caller to offer support for calling these "hidden" services, but it should know enough to ignore them without producing errors.

[wjwwood]

I proposed https://github.com/ros-visualization/rqt_service_caller/pull/20 to fix this.

[wjwwood]

Closed with #20