Improve cherry pick helper

[kaos]

I’m trying to create a cherry pick pr manually, figured I ought to be able to use ./build-support/cherry_pick/make_pr.sh 19116 2.17.x but that looked up the PR info just fine but the changeset is off.. little info in the sources in terms of docs/comments and I’ve not grokked the code apparently 😛

We should make the script error if the PR doesn't have the cherry pick failed label, with helpful message

https://pantsbuild.slack.com/archives/C0D7TNJHL/p1692661381715759