Servlet error pages do not work when Response.writeError is called before entering the ServletChannel

**Jetty version(s)**
12.0.x, 12.1.x

**Description**

From places like `Authenticator`s the `Response.writeError()` may be called before the request has entered the `ServletChannel`. 

This will end up in the servlet `ErrorHandler` which may dispatch to a servlet, but it is not running in the `ServletChannel` so this does not work as intended.



