for envoy to serve as an external proxy, I believe it needs the ability to return custom error responses. For example, instead of returning a generic http body Service Unavailable, I can have it return an error msg consistent with the formatting of my public facing api, such as:

{
  "error": {
    "type": "service_error",
    "status_code": 503,
    "message": "The service is temporarily unavailable."
  }
}

to do this in haproxy, one adds something like errorfile 503 /etc/haproxy/503.json to the config. Can we add similar functionality to envoy? (to use envoy right now for my public facing api, I actually have to put haproxy in front to serve custom error files based upon envoy's response codes.

As for the best place to put it, since it is an http attribute, perhaps it should belong as an http filter?

related to #378 (for what it is worth, I don't think envoy should serve general static files)