In the recent fix #1895, the `clientError` event was removed from the
list of events proxied from the underlying server object to the restify
Server. That fix was needed because merely adding a listener to the
underlying server for the `clientError` event prevents the default
behavior of returning an appropriate error code and closing the socket.
So by setting up a listener to proxy the event we were preventing that
default behavior, leading to hanging/leaked requests.

But! We should still support proxying `clientError` events if restify
users have added a listener for it. This enables logging or other custom
behavior. Restify users adding a `clientError` listener take on the
responsibility of sending an appropriate error and closing the socket,
exactly as noted in the Node.js docs for the `clientError` event.
https://nodejs.org/docs/latest-v16.x/api/http.html#event-clienterror

This change ensures we add a listener for `clientError` to the
underlying server object if and only if a listener for `clientError` has
been added to the restify server.