Returns a textproto.Error

It easier for the client to extract the code and message with type assertions.

ftp.go

@@ -130,7 +130,7 @@ func (c *ServerConn) cmdDataConn(format string, args ...interface{}) (net.Conn,
 	}
 	if code != StatusAlreadyOpen && code != StatusAboutToSend {
 		conn.Close()
-		return nil, os.NewError(fmt.Sprintf("%d %s", code, msg))
+		return nil, &textproto.Error{code, msg}
 	}
 
 	return conn, nil