oidc-exchange: specialize error on PRs from forks

[woodruffw]

This specializes the token retrieval error handling a bit, providing an alternative error message when the error cause is something that we know can't possibly work due to GitHub's own restrictions on PRs from forks.

Closes #202.

See https://github.com/python-pillow/Pillow/pull/7616.

[webknjaz]

@woodruffw I added one last suggestion for a thing that wasn't obvious when I was checking it out from the mobile client. Looking at your commits, I take it you expect me to use the squash strategy, right?

[woodruffw]

I added one last suggestion for a thing that wasn't obvious when I was checking it out from the mobile client. Looking at your commits, I take it you expect me to use the squash strategy, right?

Thanks! And yeah, that's what I was thinking -- that being said, I can also squash this locally and force-push if that's preferred (including going forwards).

[webknjaz]

Thanks! And yeah, that's what I was thinking -- that being said, I can also squash this locally and force-push if that's preferred (including going forwards).

I usually prefer nice atomic commits and a natural Git merge mode (not squash or rebase) to preserve history. But if that's not observed, I'm okay with selecting a squash mode, as long as it's better and/or requested. If it's not requested, I'll use my best judgement but I still wanted to clarify just in case you have your own preferences. The reason is that the responsibility for writing a good commit message shifts towards the merger in the case of squash compared to merge where the contributor would compose commit individual messages and the maintainer would only be adding something to the merge commit message, not touching the atomic ones.