moleimaging.biggest_contour: better error messages
moleimaging.biggest_contour: better error messages

Make it easier to distinguish between the situation where there are no
contours and a situation where the contours don't have enough vertices.

After running into a problem with "mel-debug gen-repo" generating a
'skin area' that didn't have enough vertices, make it quicker to debug
the same situation again.

aevri/mel,aevri/mel