fix: prevent MQTT from setting connected state on failed connections

When the MQTT broker rejects a connection (bad credentials, not
authorized, etc.), _on_connect still unconditionally sets connected=True,
subscribes to topics, and calls _set_initial_topics(). This causes
publish() to attempt sending messages to a broker that rejected the
connection.

Add an early return after logging the connection error so the client
correctly remains in a disconnected state.

frigate/comms/mqtt.py

@@ -199,6 +199,7 @@ class MqttClient(Communicator):
                     "Unable to connect to MQTT server: Connection refused. Error code: "
                     + reason_code.getName()
                 )
+            return
 
         self.connected = True
         logger.debug("MQTT connected")