Show a warning icon in the tray if there are connection issues

[Phlamethrower]

I just wasted half an hour trying to debug a network connectivity issue that turned out to be a problem on my end, because ZeroTier had quietly gone into the "Offline" state. I'd have probably spotted this a lot sooner if ZeroTier had told me about the problem, e.g. by showing a warning/error symbol on the tray icon, or by showing a system notification.

It'd probably also make sense if it showed a warning/error symbol for any issues with the networks you're meant to be connected to (Unless it already does this?)

I'm using the version of the UI that was shipped with the macOS version of ZT 1.8.4