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.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9445 cda61777-01e9-0310-a592-d414129be87e