I tested the behavior of the following servlet with different servlet containers.

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    req.startAsync(); 
    throw new NullPointerException();
}

Tomcat, Jetty and Glassfish respond with status code 500.
Wildfly 8 and 9 responds with status code 200 and a regular error page showing the stacktrace.