When given a non RPC handler callback, it didn't work.

RpcError are now reserved to rpc protocol, the RpcConnection uses the new
RpcConnectionError class.

On connection, the new sjRpc work in compatibility mode with the old one. The
label is not defined on frames, and a capabilities special message is sent to the
peer. Old sjRpc ignore this kind on message (but log a warning "malformed
message"), unlike new version which disable this compatibility mode on
receipt.