MQTT is runing without WLAN

[odon4t4]

We've had the situation that the MQTT client was running but before the WiFi access got a problem. So nothing at all was send out.

We should check if a connection was established and then sending out the stuff via MQTT

[wegendt-bosch]

This needs some thought. We can emit an assert(false) at the end of event handlers if exception != NO_EXCEPTION or we can modify all enqueues to enqueue the same event handler and pass the actual event handler as a pointer. The fake wrapper then calls the handler, checks its return and does something if an exception happened. Another possibility would be that the SDK adds a defined way to react to errors returned by event handlers. This is the hardest and nicest solution.

See also #34 for an RFC on this.