Need to Send HTTP-error-500 when an Exception occurs

I have designed a RESTful backend using struts2-rest-plugin. My front-end is Ember.js CRUD app.

When I try to create a new record with duplicate ID my server is showing an exception.

How do I send it as HTTP-error-code-500 to my ember.js app?

In case of Struts I just needed to return DefaultHttpHeaders.

 public HttpHeaders create() {
        try{
            employeesService.save(model);
        }
        catch(Exception e){
            System.out.println(e);
            return new DefaultHttpHeaders("DuplicateId").withStatus(409);
        }
        return new DefaultHttpHeaders("success")
            .setLocationId(model.getId());
    }