Distinguish between localHangup, remoteHangup, and call failure.
This allows us to put CallKit in the proper state, ready to receive new
calls without having a backlog of phantom calls which haven't been
properly removed.
Note the "call error" occurs at the point ICE fails, which takes a
while. Anecdotally, like 10 seconds, which feels like a long to be
talking into the ether.
I briefly considered failing at 'disconnected', which happens much
sooner, but that's actually a recoverable state. E.g. if you toggle
airplane mode you can see that you bounce into `disconnected` and then
back to `connected`, so I don't think we'd want to fail the call as long
as WebRTC considers it "recoverable".
// FREEBIE
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