split-cmd.py: change -N to --noop (which actually exists) in error messages.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
[rlb@defaultvalue.org: adjusted commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>