compel: tell user what's wrong with usage

In case our command line is not good, it's not enough to just show
usage info -- it is much better to explicitly say what's wrong.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>