Catch WebSocket connection errors

[Kevin-Jin]

Is there a particular reason why we don't catch errors resulting from new WebSocket(...) in Rserve.create()? It's a pretty easy fix to add the line

socket.onerror = handle_error;

so that node.JS doesn't crash just because of a connection error.

[cscheid]

Not off the top of my head. Thanks for looking into it.

Pull requests with test cases always welcome :)