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>

builtin/log.c

@@ -1050,7 +1050,7 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 
 	if (!use_stdout &&
 	    open_next_file(NULL, rev->numbered_files ? NULL : "cover-letter", rev, quiet))
-		return;
+		die(_("failed to create cover-letter file"));
 
 	log_write_email_headers(rev, head, &pp.after_subject, &need_8bit_cte, 0);