private boolean tryHasResponded() {
            synchronized (this) {
                if (hasResponded) return true;
                hasResponded = true;
            }
            return false;
        }
Invoked to write an error response when the worker pool is overloaded.
A subclass may override this method to define a custom response.
container-core/src/main/java/com/yahoo/container/jdisc/ThreadedRequestHandler.java