merge-on-green: The bot should post more details if automerge fails

[plamut]

Recently it happened that the bot did not merge a PR, even though the automerge label was applied, the PR was approved, and all CI check passed. When it works, it usually merges a PR within a few minutes after the checks pass.

In that particular case the bot just posted a comment and removed the automerge label, after which I merged the PR manually without making any changes to it, I just clicked the Merge button.

It would be helpful to know why the bot failed to merge a PR, e.g. was it just an internal glitch or was there something else that reviewers should be aware of.

[dandhlee]

+1 for being a lot more verbose to failures.

As well, it takes way too long to find out 6 hours after that the PR can't be merged. If the bot can be a bit more verbose right away (and update the comment as the PR updates itself with checks) as to why it's not working, that'd be great. For those who doesn't like the bot to be noisy, an automerge:quiet label that does the exact same work but without being verbose would make them happy as well :)