The single most common error in CI for new contributors currently is
that the commit is not signed, or signed with a key that can't be
fetched from the OpenPGP or Ubuntu keyservers.

Make the error messages more clear. In particular if a commit was not
signed the CI could just exit - now it will exit with error message.

Use YAML scalar `- >` to allow multilines without extra \ syntax and the
end of each line. Use `git log --pretty="%GP"` to avoid complex `awk` and
to simplify code and to have a more robust empty variable check instead
of relying on exit code inside a pipe.

Also fix indentation of git command to be easier to read on a single
line.