Improve error message when basedir is nil

We do not support a nil basedir in r10k, but we recently discovered that folks were explicitly setting a nil basedir. This results in a TypeError for most code paths.

We should explicitly check that basedir is set and provide a helpful error message if it's not.

See https://github.com/puppetlabs/r10k/issues/1188 and https://github.com/neillturner/kitchen-puppet/issues/214 for context.