Initiate connection from RFB constructor

Initiate connection from RFB constructor

We need to do this in order to safely attach to existing WebSocket
objects. There may be message events already pending so we must set up
our event handlers before returning.

This means we will now throw errors instead of generating "disconnect"
events on problems as the caller no longer has the opportunity to set up
event handlers.

This might have been the correct approach from the start as it mimics
how e.g. the WebSocket constructor works.