Create Custom Error Type For Peer Connections

[GGist]

Currently we stash errors like payload checks, protocol errors, block size checks, etc in io::Error. However, tokio_io::codec::Decode and tokio_io::codec::Encode would allow us to specify some wrapper error.

Currently we store informative strings in the custom error type, so its not too difficult to track down why we severed a peer connection, but if we want client to be able to easily check why we severed the connection, and ban peers based on this, we need to export some enum of all possible error types they may want to ban on (as well as a catch all io::Error variant).