allow the clock to continue on loss of WiFi

[kolalde]

We'll lose the ability to keep in NTP sync, report temp, WebServer, .... but it shouldn't just go into an endless MQTT retry loop, or reboot.

If the clock reboots (and has no wifi) it drops inthe the WiFiManager, I'm generally OK with that.

But if we're running we need to:

• keep running
• suffer wifi failures
• reconnect when possible
• lightly alert the user (but still display the time)