Mines's error signalling is highly asymmetric: if you erroneously
believe a square to be empty, you find out instantly and lethally,
but if you erroneously believe a square to be full you can
occasionally (when it doesn't cause a complementary square to be
assumed empty) not notice until you find at the very end of the game
that you're one mine heavy. To help with this, here's an error
highlighting patch: any number square surrounded by an excess of
flags will now light up red. This should be an unintrusive change,
because it will never happen unless you make a mistake.

[originally from svn r6580]