Add specific exceptions for CONNACK in MQTT

Motivation:

http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html#connack
In MQTT 3.1, MQTT server must send a CONNACK with return code if CONNECT
request contains an invalid client identifier or an unacceptable protocol
version. The return code is one of MqttConnectReturnCode.
But, MqttDecoder throws DecoderException when CONNECT request contains
invalid value without distinguish situations. This makes it difficult
for codec-mqtt users to understand what is wrong.

Modifications:

Added exceptions for client identifier rejected and unacceptable
protocol version. MqttDecoder will throw those exceptions instead of
DecoderException.

Result:

Users of codec-mqtt can distinguish which is invalid when CONNECT
contains invalid client identifier or invalid protocol version. And, users can
send CONNACK to clients with correct return code.