Unit Motion - Stop when targets have an invalid position.
Previously, unitMotion had no code that checked particularly if the target was still in the world.
When the target moved out of the world, unitMotion would follow the path to its last known position, then send a "MoveSucceeded" message once there.
Following rP22366, this message was no longer sent. Thus unit would follow their path to its last waypoint and stay there, unable to carry on or finish the order. UnitMotion now explcitly sends a "MoveFailed" message.
This still changes behaviour from A23, requiring further revisions to UnitAI (see D1992 for one such case).
Minohaka tested an earlier version of this change (which incorporated D1992) and accepted it.
Differential Revision: https://code.wildfiregames.com/D1979