Add returncode to subprocess.CalledProcessError output

[jdillard]

This is for slightly better output as discussed in #69. It appears not all return codes have stderr output associated with them, so this will help in those rare cases.

Apparently there are ways to get more information about the return code, such as this line in subprocess.py, but I didn't know how finicky that would be and I left it as the bare return code.

Fixes #69

[modelmat]

@jdillard Looks like the line length is too long so if you can fix that I'll merge this.

[jdillard]

Oops! I think I got it right this time.

[modelmat]

Now to figure out how to push a release... I've forgotten.

[jdillard]

Ha I know the feeling! I have to put a MAINTENANCE.md file in my projects with instructions for myself so I don't forget (there is probably a better way though).

[modelmat]

Should be released now. Ignore the fact CI says it failed to release, it seems to upload it twice for unknown reason and then fail the second.

[jdillard]

I see it https://pypi.org/project/sphinxcontrib-drawio/. You rock! Thanks so much!