Improve handling of sudden client disconnections
Improve handling of sudden client disconnections

This prevents sudden client disconnections (such as refreshing a browser
window), occassionaly generating an error when trying to acknowledge the
disconnection attempt due to the underlying socket already being closed.

From a consumer of WebSockets.jl this should look similar to a regular
disconnection by throwing WebSocketClosedError instead.

JuliaWeb/WebSockets.jl,JuliaWeb/WebSockets.jl