add: mark --chmod error string for translation

add: mark --chmod error string for translation

This error message is intended for humans, so mark it for translation.
Also use error() instead of fprintf(stderr, ...), to make the
corresponding line a bit cleaner, and to display the "error:" prefix,
which helps classifying the nature/severity of the message.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>