pid1: add new mode systemd.show-status=error and use it when 'quiet' is passed

systemd.show-status=error is useful for the case where people care about errors
only.

If people want to have a quiet boot, they most likely don't want to see all
status output even if there is a delay in boot, so make "quiet" imply
systemd.show-status=error instead of systemd.show-status=auto.

Fixes #14976.