pack-write: die on error in write_promisor_file()

pack-write: die on error in write_promisor_file()

write_promisor_file() already uses xfopen(), so it would die
if the file cannot be opened for writing. To be consistent
with this behavior and not overlook issues, let's also die if
there are errors when we are actually writing to the file.

Suggested-by: Jeff King <peff@peff.net>
Suggested-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>