Update ErrorPageFilter to only handle errors when `response.sendError`
has been called. This should allow custom @ExceptionHandlers to
completely handle errors and return custom status codes without
triggering the "Cannot forward to error page" log message.

The Javadoc for sendError states:

  "The server defaults to creating the response to look like an
   HTML-formatted server error page containing the specified message"

Where as setStatus states

  "This method is used to set the return status code when there is
   no error "

Fixes gh-2745