zdtm: don't hide errors in cgroup02.hook

rmdir is executed for non-existent directories, so we don't check
an exit code of this operation.

This patch executs rmdir only for existent directories and check
an exit code of rmdir.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>