Reveal git error messages on clone/fork fail, enforce Github username

[louisrli]

Hi all, I'm teaching a coding bootcamp for Re:Coded that uses learn.co. Some students are getting tripped up by obscure error messages about being unable to clone, and it's hard for us to diagnose the issue based on the current error message

This pull requests does two things:

1. When there is a Git error in cloning/forking, it prints out the error to the console (not to the logger) so that students can show it to instructors.
2. In initializing the lesson, it enforces that @organization can be parsed and is non-empty as a result. If it is empty, it suggests that the user link a Github account (perhaps there's a better place to do this, the maintainer can decide)

Note that there is an additional bug that's not being addressed, which is that the "forking logic" passes successfully even when there is no Github account, but despite nothing actually being forked. For this, I've filed #40

(The root issue is the setup documentation: none of the Mac, Linux, or Windows instructions say that linking your Github account is required, e.g., this link does not say you must link your Github account. https://github.com/learn-co-curriculum/wsl-setup)

[louisrli]

can someone look at this pr

[louisrli]

ping