Merge fails with a cryptic git push error when GitHub branch protection rules block it

Quite often, when I try to merge a pull request using Rultor, I get a confusing error like this:

git push origin master
remote: error: GH013: Repository rule violations found for refs/heads/master.
remote: - A conversation must be resolved before this pull request can be merged.
To github.com:objectionary/lints.git
 ! [remote rejected]   master -> master (push declined due to repository rule violations)
error: failed to push some refs to 'github.com:objectionary/lints.git'

The problem is that the error comes from git push at the very end of the merge process — after Docker is started and the full build is run. At that point, the error message is raw and unhelpful. In this particular case, I had to figure out on my own that the rejection was caused by an outdated (but unresolved) review thread left by a reviewer on an already-superseded commit.

GitHub branch protection rules can block a merge for various reasons: unresolved review conversations, missing required approvals, failing required status checks, and so on. The git push rejection message does not tell me which specific rule was violated or what I need to fix.

You can find a real case here.