Issue Description:

I've noticed in the past that update-checkout gives terrible "diagnostics" if `git clone` fails: basically, it just crashes and lets Python print its normal exception stacktrace, which is really difficult to find useful information in (even for me, much less someone who doesn't know what to look for). It also seems to fail quite badly if it's not in a checkout directory, e.g. if someone's trying to build from a source tarball; we don't necessarily need to support that, but we should give more useful feedback than a Python exception.