Dubbo throws IoException with error message while decoding fail

[qixiaobo]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.6.6
• Operating System version: *nix
• Java version: 1.7+

Expected Result

throw new IOException("Unknown result flag, expect '0' '1' '2' '3' '4' '5', get " + flag);

Actual Result

throw new IOException("Unknown result flag, expect '0' '1' '2', get " + flag);

since dubbo add new type for class dubbocodec for issue #1843
we should also change the exception message .

For example, if dubbo provides new protocol version(higher than 2.0.2) .then the high version dubbo invoke the lower version dubbo[2.0.2] with no support codec ,then the lower version dubbo throws ioexception with incorrect message.
So we need fixit