Better error messages when cropping too big slices

Hi. Cropping slices that were vertically too big made `chunky_png` complain that `Image width is too small!`, when it's actually the height that is too small. I also think it's better to explicitly say that it's the original image that is too small, not the slice - when I saw those exceptions, my first idea was "Hey, I must be putting negative numbers into the third or fourth argument".