Reset cancel event

[rwb27]

I think it wouldl be a good idea to reset the cancel event after aborting a move and raising an exception. This would allow, for example, an action to move back to the starting position when it's cancelled (without using the block_cancellation argument, which I feel should be used sparingly).

[rwb27]

This might be better done by the calling code - i.e. when you handle an InvocationCancelledError, you should reset the event?