finish() support for CI environments

[Zethson]

ln.finish() errors with the "must save notebook" error when run on CI. I got confused earlier why this wasn't working to have to learn that it simply doesn't support CI because we cannot manually save the notebook there.

1. I would suggest that we document this behavior better.
2. I would suggest that we detect CI environments (environment variables) and provide a different error message stating that this is not supported.