Add a meaningful error code if RealtimeChannel.PublishAsync times out

[marto83]

Currently we return an error code of 0 if PublishAsync doesn't complete in 10 seconds. However the error code and status code passed back to the client is 0 so it's doesn't mean anything.

Here is the code in question: https://github.com/ably/ably-dotnet/blob/e2f834d3f287f06b61a9d8e55da65bbba71a4bce/src/IO.Ably.Shared/Realtime/RealtimeChannel.cs#L513

┆Issue is synchronized with this Jira Task by Unito