Improve error messages for users on building

Build fails if there is no requirements.txt in root directory.

[choldgraf]

Let’s keep a running list of situations for which we want an error message. E.g.:

• If no github URL exists for the variables a user has given, right now nothing happens, we should instead print a “not found” error.

[yuvipanda]

We’re doing a lot better now, am going to close this in favor of more explicit single issues.

Thanks for filing and working on fixes, @willingc!