Dial: Catch NotPrimaryMaster & return custom error

After initial handshake a NEO node checks the identification of its peer
by sending the 'RequestIdentification' packet. In case the peer is a
secondary master it responds with 'NotPrimaryMaster'. Before this patch
'Dial' ignored the 'NotPrimaryMaster' packet and simply returned a
general error. Now - after this patch - 'Dial' returns an instance of
'proto.NotPrimaryMaster' (which implements 'Error').
This helps a caller to correctly handle the secondary-master-case, which
otherwise is impossible to differentiate from any other error
possibility.

go/neo/xneo/connect.go

@@ -135,17 +135,37 @@ func Dial(ctx context.Context, typ proto.NodeType, net xnet.Networker, addr stri
 		// start, and if peer sends new connection in that window it will be rejected.
 		//
 		// TODO thinking.
-		err = link.Ask1(reqID, accept)
+
+		conn, err := link.NewConn()
+		if err != nil {
+			return err
+		}
+		defer conn.Close()
+
+		err = conn.Send(reqID)
 		if err != nil {
 			return err
 		}
 
-		if accept.NodeType != typ {
-			// TODO send Error to peer?
-			return fmt.Errorf("accepted, but peer is not %s (identifies as %s)", typ, accept.NodeType)
+		notPrimary := &proto.NotPrimaryMaster{}
+		nerr := &proto.Error{}
+
+		which, err := conn.Expect(nerr, notPrimary, accept)
+
+		switch which {
+		case 0:
+			return nerr
+		case 1:
+			return notPrimary
+		case 2:
+			if accept.NodeType != typ {
+				// TODO send Error to peer?
+				return fmt.Errorf("accepted, but peer is not %s (identifies as %s)", typ, accept.NodeType)
+			}
+			return nil
 		}
+		return err
 
-		return nil
 	})
 	if err != nil {
 		return nil, nil, err