Forking works even if Github user name is not set

[louisrli]

In lesson_downloader.rb, there's a line of code that says:

fork_repo if lesson.use_student_fork

If you don't have a Github username linked to your learn.co account, this line does not crash; nothing goes wrong, however, nothing is actually forked, because it doesn't know where to fork to.

This should be a fatal error or somewhere upstream should enforce that the Github username is linked if it attempts to fork.

To be clear, to replicate this bug:

1. Sign up for a learn.co cohort
2. Do NOT link your Github
3. Run learn open <some lesson that attempts forking>
4. The fork code will run without an error
5. No repo will actually be forked