sequencer: report when noop has an argument

The noop command cannot accept any argument, but we never told the user
about any bogus argument. Fix that.

while at it, mention clearly when an argument is required but missing
(for commands *other* than noop).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>