Consider changing the default behavior of on_cancelled()

[sarcasticnature]

Currently the implementation of BtActionNode::on_cancelled() returns BT::NodeStatus::SUCCESS. I have found this to be counter-intuitive, as the goal has (by definition) not been successfully completed, and may often require overriding to return a failure.

I would like to get @fmrico's opinion on changing the default behavior to BT::NodeStatus::FAILURE to better reflect what is actually happening.

[Splinter1984]

If I understand correctly, it would not be better in this case to return BT::NodeStatus::IDLE?

[fmrico]

Well, the current semantic is BtActionNode::on_cancelled() returns BT::NodeStatus::SUCCESS if the action could be succesfully cancelled. I think it is reasonable, isn't it?