CI: never fail, improve error messages and post a comment

As mentioned in the title this has:

• No red crosses, only green checks (except if posting a comment fails… to avoid silent problems)
• No scary e-mails to contributors, like "your pipeline failed"
• Comments on commits when something is wrong, that explain the issue with a tutorial to fix, and help to reach maintainers

This is my first actual contribution to the CI ;)