Grouped source lines in output aren't sorted numerically.

[lagru]

Citing @mkcor:

I've noticed one possible area for improvement in how errors are listed: They appear in alphanumerical order, so the line numbers are not sorted as numbers. It's not a big deal, but it's more natural to go through a file starting from the beginning.
See:

src/skimage/exposure/exposure.py:180
src/skimage/exposure/exposure.py:47
src/skimage/exposure/exposure.py:610
src/skimage/exposure/exposure.py:619