Improve connectedness-error highlighting in Range.

Improve connectedness-error highlighting in Range.

The previous approach of scanning the grid by depth-first search was
fine for deciding whether it was connected, but not so good for
pointing out where the mistake was in the grid. Replaced that code
with a dsf-based version, which identifies all connected components so
that an easy followup pass can highlight all but the largest as
erroneous.

[originally from svn r10223]