Error checking in Pattern, which was until now the only remaining game

Error checking in Pattern, which was until now the only remaining game
that _ought_ to have it but did not.

I've tried to implement it before and found that the most obvious
approach was so effective as to constitute a spoiler, so this is a
deliberately weakened approach which in a bit of play-testing seems to
be a more sensible balance. It won't necessarily tell you at the very
instant you put a foot wrong, but it will at least ensure that (my
usual minimum standard) once you've filled in the whole grid you will
either have seen a victory flash, or an error indicator showing you
why not.

[originally from svn r9445]