If the queue is full and a new move is queued, panic!
Introduce a new error code QUEUE_FULL to help diagnose situations where
the queue is handled improperly: likely one of the state machines not
waiting for the previous actions to finish.
PulseGen::PlanMove returns a boolean if the queue cannot be moved.
We could extend this to Motion::PlanMove, however all moves would then
have to check for this. Having a global check such as this ensures
we never ignore such situation.