wrapper.c: add and use fopen_or_warn()

When fopen() returns NULL, it could be because the given path does not
exist, but it could also be some other errors and the caller has to
check. Add a wrapper so we don't have to repeat the same error check
everywhere.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -666,7 +666,7 @@ static int is_expected_rev(const struct object_id *oid)
 	if (stat(filename, &st) || !S_ISREG(st.st_mode))
 		return 0;
 
-	fp = fopen(filename, "r");
+	fp = fopen_or_warn(filename, "r");
 	if (!fp)
 		return 0;