Consider adding return code of "Authentication failed"

There is no return code on CONNACK and DISCONNECT for "Authentication Failed". The general guidance is to use "Not Authorised". This seems a little confusing as the Client can't be authorised as it's not yet authenticated. This might make debug more difficult and lead to some implementation confusion.

We do make the distinction elsewhere in the spec e.g.,

3.1.3.4 User Name
If the User Name Flag is set to 1, the User Name is the next field in the Payload. The User Name MUST be a UTF-8 Encoded String as defined in Section 1.5.4 [MQTT-3.1.3-10]. It can be used by the Server for authentication and authorization.