Handle all possible MQTT disconnect exceptions

[FredericMa]

This PR improves the handling of MQTT disconnects. Previously, only Mqtt3DisconnectException and Mqtt5DisconnectException were handled.

• ConnectionFailedException if a connect attempt failed.
• Mqtt3ConnAckException or Mqtt5ConnAckException (depending on the MQTT version of the client) if the ConnAck message contained an error code, which means that the connect was rejected.
• ConnectionClosedException if the connection was closed without sending a Disconnect message (use getSource() to determine if the server or the client closed the connection).
• Mqtt3DisconnectException or Mqtt5DisconnectException (depending on the MQTT version of the client) if the connection was closed with a Disconnect message (use getSource() to determine if the server, the user or the client sent the Disconnect message).

This PR probably fixes #39 and #40.

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[TheTimeWalker]

Trusting your changes. Thank you for looking into this bug!