stbt.match: Better error message when region doesn't overlap frame
stbt.match: Better error message when region doesn't overlap frame

Previously you'd get:

      File "_stbt/match.py", line 262, in match
        result = next(_match_all(image, frame, match_parameters, region))
      File "_stbt/match.py", line 377, in _match_all
        crop(frame, region), t, mask,
      File "_stbt/imgutils.py", line 79, in crop
        % (frame.shape, region))
    ValueError: frame with dimensions (60, 200, 3) doesn't contain None

Now you see the actual dimensions of the region you specified, instead
of "None".

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester