Very fiddly corrections to the loop highlighting. ERRSLASH means the

Very fiddly corrections to the loop highlighting. ERRSLASH means the
slash in this square is red, so it does indeed imply that some of
the edge markings are also red; but it doesn't mean _all_ the edge
markings must be red. So instead of assuming ERRSLASH implies all
edge error markers, we set the correct set of edge error markers at
the same time as setting ERRSLASH, at which point we know which kind
of slash it is so we know which ones to set.

[originally from svn r6175]