Throw exceptions from RFB constructor

Previously, if an error was thrown from the Display constructor
in the RFB constructor, we would attempt to use `RFB#updateState`
to handle this.  However, `RFB#updateState` attempts to close
the WebSocket connection, which doesn't exist at this point.

In the constructor, it's probably just better to raise an exception
instead (making sure to clean up anything relevant).

Fixes #460