edge of map feedback

edge of map feedback

When testing Planes of Air and Water, I found it odd that trying to
move off the edge of a level uses a turn but provides no feedback.
If 'mention_walls' is On, report that you can't go any farther in
whichever direction you're trying to move.  Moving diagonally is
only blocked in one of the two combined directions unless you're in
the very corner, so if you try to move southwest while in the middle
of the bottom row, for instance, it says you can't go farther south
rather than southwest.