Mistake in course path at setup creates unfixable state

If the user enters the wrong course url for gitlab in the course configuration, then proceeds with the setup, it obviously fails (like it should) with an error about non existing group url. However, this puts the course edition in an unfixable situation where all buttons give error pages and there is no way to revert the setup.

There should be some way to revert the setup (or checks to prevent getting too far into the process).