format-patch: notice failure to open cover letter for writing

format-patch: notice failure to open cover letter for writing

The make_cover_letter() function is supposed to open a new file for
writing, and let the caller write into it via FILE *rev->diffopt.file
but because the function does not return anything, the caller does not
bother checking the return value.

Make sure it dies, instead of keep going with a NULL output
filestream and relying on it to cause a crash, when it fails to
open the file.

Signed-off-by: Junio C Hamano <gitster@pobox.com>