check_mirrors should have an option to report success, too
While doing release management, it is often needed to check the status of mirrors. In particular, before testing IUKs, "most" of them must be update. However, ./check-mirrors.rb --channel ${DIST:?} --allow-multiple --fast tails-amd64-${VERSION:?}
(the command suggested in the doc) isn't very useful, because it only reports errors.
As a release manager, I want to know the ratio of mirrors that have the update.
Proposal: have an option to both report good mirrors and failed mirrors clearly