We recently added support for routes to be able to produce responses, other than original MVC forwarding to a view. This addition meant that a route could specify the media types it produces, and a client could specify the desired media types using the HTTP Accept header, and get back a response body with that type.
But this only worked for normal routes and not for error routes which is what AEROGEAR-515 has added.

You can now specify that your error routes produce different types:

route()
      .on(Exception.class)
      .produces("text/html", "application/json")
      .to(Error.class).index(param(Exception.class));

route()
      .from("/throwException")
      .on(RequestMethod.GET)
      .produces("text/html", "application/json")
      .to(Error.class).throwException();

This would enable you to produce a html response by accessing throwException in aerogear-controller-demo.

You can also specify that you want the response as application/json and calling the same route as above:

curl -i -H "Accept: application/json" -X GET  "http://localhost:8080/aerogear-controller-demo/throwException"

GET Exception application/json:
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/json;charset=UTF-8
Content-Length: 28
Date: Thu, 03 Jan 2013 06:52:56 GMT

"{exception:Demo Exception}"