Specifying a handler module that does not implement the handler behaviour fails unintuitevly

[mobileoverlord]

When setting up a connection for the first time, it might be easy to overlook that the handler points to a module that was not fully configured. This fails silently and causes the system to "appear to be responsive and connected" but it is not. pings and publish messages will hang forever.

[gausby]

I will make sure the MQTT 5 branch will validate the tortoise handler before using it. I know I have a todo in the code about this in the current version.