builtin: change IError msg and code to methods + fix vlib, add a deprecation notice for the old usages (#13041)

vlib/net/websocket/websocket_server.v

@@ -122,7 +122,7 @@ fn (mut s Server) serve_client(mut c Client) ? {
 	}
 	s.setup_callbacks(mut server_client)
 	c.listen() or {
-		s.logger.error(err.msg)
+		s.logger.error(err.msg())
 		return err
 	}
 }