Avoid expensive path-finding when the transport already sailed away

This aborts earlier on `EnterTransport` orders. I especially tried to avoid `MoveAdjacentTo` that will trigger `PathSearch` and `PathFinder` and are worse-case scenarios for impossible paths that happen regularly when trying to board ground units into water transport. I was assured in my guess when I found a forseeing TODO comment warning that this can happen and has to be avoided.

I believe this will drastically improve performance on @wuschel's giant island oil derrick capture maps where you are constantly micro-managing transporters and you tend to forget more and more soldiers that will try to path through the whole (giant) map and this stacks up while you are amassing your armies.