If Settler AI can't complete the next step in its path, it gives up and stays where it is forever #213

This definitely occurs if the settler's destination is blocked.

How to reproduce:

• Figure out where an AI is sending its settler by looking at the logs
• Send a unit to that tile and block it
• The AI Settler will hit the Exception that causes it to print "Could not get next part of path for unit", with unit info.
• The AI Settler will just stay on its current tile forever

The immediate cause of this is TilePath to the destination running out of locations after the settler tries (but fails) to move to the occupied tile; the next time it throws an exception due to the TilePath queue being empty, and then it just is stuck.

At a higher level, though, this is because we aren't validating that the path (or even just the next step in the path) remains valid. But to fix that, we'll need to enhance the pathfinding algorithm to not route the unit through a rival AI's unit. And it probably doesn't make sense to invest in making BFS better to address that, thus this should not be fixed before #100 unless it becomes more important for some reason.

We might also have odd behavior if a non-final step in the path is blocked, as then the unit likely tries to move to a non-adjacent tile the next turn, but I haven't verified that exact scenario.

Leaving off of Carthage for now as I'm not aware of this being common, and we have enough Settler AI enhancements to make already.