box/console: fix abnormal exit after unknown command

When invalid command is passed we should send an error message to a
client. Instead a nil dereference occurs that causes abnormal exit of a
console.

This is the regression from 96dbc49d
('box/console: Refactor command handling').

Reported-by: Mergen Imeev <imeevma@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>
(cherry picked from commit ada8c97c)