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.


git-svn-id: svn://svn.tartarus.org/sgt@10223 cda61777-01e9-0310-a592-d414129be87e