systemActions: Don't log error when cancelled

systemActions: Don't log error when cancelled

If the logout/reboot/shutdown actions are available, we expect them
to succeed. Unfortunately that changed in the light of systemd
inhibitors (that we don't know about in the shell, and that prevent
those actions now).

While it would be good to fix the underlying issue, we can at least
improve the error reporting in that case and not log the error when
the operation was cancelled: We don't know the underlying reason,
unlike gnome-session which should have logged a more helpful message.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8749
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3927>
(cherry picked from commit d3f3f4e80d338133a2083dd5c267ed456b86303d)