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);
 

t/t4014-format-patch.sh

@@ -589,6 +589,12 @@ test_expect_success 'excessive subject' '
 	ls patches/0004-This-is-an-excessively-long-subject-line-for-a-messa.patch
 '
 
+test_expect_success 'failure to write cover-letter aborts gracefully' '
+	test_when_finished "rmdir 0000-cover-letter.patch" &&
+	mkdir 0000-cover-letter.patch &&
+	test_must_fail git format-patch --no-renames --cover-letter -1
+'
+
 test_expect_success 'cover-letter inherits diff options' '
 	git mv file foo &&
 	git commit -m foo &&