Handle TLS handshake failures

[muep]

It seems that if the MQTT broker rejects the client certificate, this is pretty silently ignored. The MQTT client may then seemingly publish messages, but these go nowhere.

Also other kinds of errors may be unhandled here.

[muep]

Looks like one common way to encounter a problem is to start with up with a badly out-of-sync clock. There is not much that can be done as long as this is the case, but the system should recover when the real-time clock is later corrected.