Highfive tries to assign reviewers not in rust-lang org

[pietroalbini]

While debugging the "highfive does not apply reviewers sometimes" bug in #rust-infra, we discovered those problems happens when the bot tries to assign a reviewer that's not in the org. This is because GitHub returns a 422 status code in those cases, and the set_reviewer function doesn't handle that.

One of those reviewers is @rkruppe, which is set in the config file for the rust repo, but the other is brson, and we don't know why highfive tries to assign him since there is no mention of him in the repo.

Other than adding @rkruppe to the org or removing them from the config file, we should patch highfive to handle the case of a reviewer not in the org. How should it handle it though @nrc?

[davidalber]

A similar idea is mentioned in #39. It's suggested there that when assignment fails Highfive should pick a different random reviewer.

On a different note, brson was in the global config file in the past. It seems unlikely that the change wasn't propagated to production, but that is one way he could mysteriously be getting selected.