Have a clearer error message when the repo_url is filled out but the repo type is not set

[mpdehaan]

It's easy to type in a repo URL for a project but leave the repo type set to None.

If we try to build a project with a repo URL filled in but the repo type is None, we should raise a clear error message, rather than just leaving an empty build directory.

This can be confusing as users will have all the info entered and wonder why their checkouts didn't happen.

Super easy fix, just needs to happen.