Detect unreachable moves and initiate backup

[joshbuker]

Currently the algorithm will continue until it runs out of moves entirely, then backup to the last critical point, however major time can be saved it there is a periodic check for unreachable moves that kicks the backup sequence off earlier. Alternatively, if there is a way to detect unreachable moves that doesn't significantly impact per-move performance, it would be most efficient to mark the move that creates an unreachable area as failed, then begin backup.