There is minimal useful debug information when the git run by python -m towncrier.check fails

[exarkun]

A poor interaction between towncrier and TravisCI resulted in this behavior:

codechecks runtests: commands[7] | python -m towncrier.check
Traceback (most recent call last):
  File "/opt/python/2.7.14/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/opt/python/2.7.14/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 95, in <module>
    _main()
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 30, in _main
    return __main(compare_with, directory)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 40, in __main
    _run(["git", "diff", "--name-only", comparewith + "..."], cwd=base_directory)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 23, in _run
    return check_output(args, **kwargs)
  File "/opt/python/2.7.14/lib/python2.7/subprocess.py", line 219, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['git', 'diff', '--name-only', u'origin/master...']' returned non-zero exit status 128

This is challenging to debug since the git output is not available. towncrier should report the git output as part of this failure report.