Gracefully handle no network connection during startup

[siku2]

This is currently a draft because it has merge conflicts with both of the other PRs.
Since this is the easiest one to resolve I would like to wait for the others to be merged before fixing the conflicts here.

Fixes #13 by catching the error during startup and displaying a notification informing the user.

Additionally it also handles the following cases:

• Connected during start of the SSDP server but not when shutting down causes the same socket error to be raised. Now it's just a warning.
• Shutting down after if the server didn't connect would cause an error during shutdown because the server attribute is never set. This is now handled.

[siku2]

It might be worth considering adding a retry loop to continuously (maybe with exponential back-off) try to connect until it succeeds.
What do you think, @enen92?

[enen92]

I am not sure network changes in kodi are propagated via the onNotification monitor interface. Probably this is something that could be looked at/tested to avoid blindingly scheduling a retry

[siku2]

I reconnected to the internet while logging all onNotification calls and I didn't see any notifications related to the network.

Maybe the System.HasNetwork condition would work though?

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Edit: In my case System.HasNetwork is true regardless of whether Kodi is connected to the internet. The condition seems to be for something else entirely.

[enen92]

That would require polling on the add-on side