When judging iptables Error, some fixed strings are used, like "iptables: No chain/target/match by that name.\n", but when we use ip6tables in ipv6 protocal, the same return message is "ip6tables: No chain/target/match by that name.\n", these two strings mismatch. And this raises some troubles like CI broken.

So I think we can add protocal to Error struct and make judgements with protocal together.