Release note generation should be non-fatal

[Mirko-von-Leipzig]

Recent release got blocked because the release notes could not generated. We should make this a non-fatal error for the release flow.

The cause was a missing PR - which github has apparently scrubbed from its servers. The merge commit exists; the PR does not - this caused the release notes tool to choke.

We should fix both of these.

[huklaa]

I'd like to take this. I checked the current open PRs and don't see an existing fix for #2520. I would keep the scope narrow: make release-note generation non-fatal in the release flow, and make the note generator tolerate a merge commit whose associated PR can no longer be resolved, with focused regression coverage for the missing-PR case. Could a maintainer please assign the issue to me before I open the PR?

[huklaa]

I'd like to work on this. Could you assign it to me?

[Mirko-von-Leipzig]

Closed by #2524