Dubbo throws IoException with error message while decoding fail (#4396)

Really have this problem

dubbo-rpc/dubbo-rpc-dubbo/src/main/java/com/alibaba/dubbo/rpc/protocol/dubbo/DecodeableRpcResult.java

@@ -128,7 +128,7 @@ public Object decode(Channel channel, InputStream input) throws IOException {
                 }
                 break;
             default:
-                throw new IOException("Unknown result flag, expect '0' '1' '2', get " + flag);
+                throw new IOException("Unknown result flag, expect '0' '1' '2' '3' '4' '5', get " + flag);
         }
         if (in instanceof Cleanable) {
             ((Cleanable) in).cleanup();