Missing lock file in main branch throws an error

[banginji]

If the lock file is created in a branch and the diff tool runs against main branch which doesn't have the lock file, then a 404 error is thrown on the http look up call and that fails the whole process

[colindean]

This might be a known issue; a bootstrapping problem. Can't diff against what doesn't exist.

I suppose we could make it display some kind of a helpful warning when the poetry.lock is missing and not fail.

For now, I think failing is fine: only codebases adopting Poetry while having this check configured will encounter this problem.