Ensured all methods in setuptools.modified raise a consistant distutils.error.DistutilsError type

[Avasam]

Summary of changes

The following code should now work as expected in all cases:

from setuptools.modified import newer_pairwise_group
from distutils.error import DistutilsError

try:
    newer_pairwise_group(...)
except DistutilsError:
    pass

I'm not entirely certain my test is correct. Hopefully it should be.

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)