core/cgroup: upgrade log level when we fail to rescope a pid

See https://bugzilla.redhat.com/show_bug.cgi?id=1973058 again:

systemd[1779]: Started Application launched by gnome-session-binary.
systemd[1779]: app-glib-liveinst\x2dsetup-1897.scope: Failed to add PIDs to scope's control group: No such process
systemd[1779]: app-glib-liveinst\x2dsetup-1897.scope: Failed with result 'resources'.
systemd[1779]: Failed to start Application launched by gnome-session-binary.
systemd[1779]: app-glib-xdg\x2duser\x2ddirs-1900.scope: Failed to add PIDs to scope's control group: No such process
systemd[1779]: app-glib-xdg\x2duser\x2ddirs-1900.scope: Failed with result 'resources'.
systemd[1779]: Failed to start Application launched by gnome-session-binary.
systemd[1779]: app-gnome-gsettings\x2ddata\x2dconvert-1902.scope: Failed to add PIDs to scope's control group: No such process
systemd[1779]: app-gnome-gsettings\x2ddata\x2dconvert-1902.scope: Failed with result 'resources'.
systemd[1779]: Failed to start Application launched by gnome-session-binary.

Since we don't show the PID anywhere, it can be quite hard to figure out what
is going on. There may be logs from the pid above or below in the log, but
we have no PID number to identify them. So let's upgrade the log from
unit_attach_pids_to_cgroup() to tell us precisely which PIDs and why couldn't
be handled.

(cherry picked from commit 7a2ba407)