core/dbus-manager: propagate meaningful dbus errors from EnqueueMarkedJobs

core/dbus-manager: propagate meaningful dbus errors from EnqueueMarkedJobs

Previously, when `systemctl reload-or-restart --marked` failed the following
error was shown:

  Failed to start jobs: Failed to enqueue some jobs, see logs for details: Invalid argument

but no details were actually logged.

This change records failure reasons in the journal to help users identify root causes:

  # journalctl -b -e
  [...]
  systemd[1]: Queuing reload/restart jobs for marked units…
  systemd[1]: Operation refused, unit klogd.service may be requested by dependency only (it is configured to refuse manual start/stop).