New end-game approach to Black Box. Instead of revealing the ball

New end-game approach to Black Box. Instead of revealing the ball
positions immediately when you make an error, the game now reveals
as little information as is necessary to prove you wrong (including
none - if an existing laser path you know about is inconsistent with
your guesses, the game will just point it out and tell you nothing
new!) and you can try again. Errors are counted in much the same way
as deaths in Mines.

[originally from svn r6152]