SubnetID parsing hides underlying error

[aakoshh]

The following is trying to parse a subnet ID copied from usage.md:

    #[test]
    fn parse_subnet_id() {
        let id = "/r31415926/t2xwzbdu7z5sam6hc57xxwkctciuaz7oe5omipwbq";
        SubnetID::from_str(id).unwrap();
    }

it panics:

---- settings::tests::parse_subnet_id stdout ----
thread 'settings::tests::parse_subnet_id' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidID'

The reason turned out to be that t2 is unparseable, because the fvm_shared::address::network::Network::current_network() is Mainnet by default which has prefix f, and if the prefix is different it returns UnknownNetwork when it tries to parse.

It would be nice if parsing a subnet ID would return the most specific error it can find, because InvalidID is a catch-all for everything: if the rootnet ID isn't a u64, or if any of the children fail to parse. It could instead return the error and the offending part.