Provide an error type for "no protocol with code"

[willscott]

Right now it's very difficult to identify if the reason a multiaddr failed is because it is corrupt, or because it uses a protocol that is newer than the parsing implementation

https://github.com/multiformats/go-multiaddr/blob/master/codec.go#L79

It would be useful to have an error type so that errors.Is can indicate that the address might be well-formed, but couldn't be parsed because it has a stanza that we don't recognize.