go/neo/neonet: _HandshakeError += Cause/Unwrap

This will be handy to test underlying error for e.g. EOF with
errors.Cause or errors.Is in case that underlying error is also error
wrapper.

go/neo/neonet/newlink.go

-// Copyright (C) 2016-2020  Nexedi SA and Contributors.
+// Copyright (C) 2016-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -65,6 +65,9 @@ func (e *_HandshakeError) Error() string {
 	return fmt.Sprintf("%s - %s: handshake: %s", e.LocalAddr, e.RemoteAddr, e.Err.Error())
 }
 
+func (e *_HandshakeError) Cause()  error { return e.Err }
+func (e *_HandshakeError) Unwrap() error { return e.Err }
+
 func handshake(ctx context.Context, conn net.Conn, version uint32) (err error) {
 	// XXX simplify -> errgroup
 	errch := make(chan error, 2)