disambiguate client vs server disconnections

[boneskull]

Regardless of whether the client or server initiates a disconnection, the onFailure callback is executed. These should be two discrete events; a client-initiated disconnection is not a "failure".

[boneskull]

I'm wrong; there's onConnectionLost which is a property that can be set after an instance has been constructed. This is rather unusual, anyhow, so #141 applies here.