compel cli: print usage to stderr in case of error

This is mainly dictated by the fact that we use stdout
from "compel *flags" in a special way, so it should not
be garbled by the usage info.

Otherwise, for example, the following code in Makefile

	CFLAGS += $(shell compel --badopt cflags)

will lead to the whole usage() output to be added to compiler flags,
which looks really really weird.

travis-ci: success for More polishing for compel cli
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>