If a client bus's poll request fails due to a transport error, or anything
that doesn't result in receiving an HTTP response, the bus hangs.
The problem is in ClientMessageBusImpl.LongPollRequestCallback.onError().
The onError() method has a short-circuit return when statusCode is zero,
but statusCode can only become non-zero if the request has completed
successfully and a valid HTTP response has been received from the server.
Thus, with some irony, onError() does not actually handle Request errors:
it only does work when called by onResponseReceived().
Since onError() short-circuits, no bus errors are broadcast on the local
bus and no reconnect attempt is scheduled. The net result is that polling
is dead and the client bus receives no more messages from the server.