custom error pages in rails 4 engine

Question

I'm creating a JSON api engine in rails 4. I'm trying to show json error messages for 404, etc. instead of the default html 404 page. So far I've navigated to /jsonforem/404 and verified that my controller is working, but when I navigate to somewhere that doesn't exist I still get the default html 404 from rails.

Here's where I'm doing routing in lib/jsonforem.rb:

require "jsonforem/engine"

module Jsonforem
  config.exceptions_app = self.routes
end

And here's my routes in the engine:

Jsonforem::Engine.routes.draw do
  resources :posts, :categories, :forums, :topics

  get 'posts/:id/children' => 'posts#childrenof'

  root 'posts#index'

  match "*path" => "errors#not_found", via: :all

end

I also tried matching from "/404", but there was no effect. Lastly, my errors controller in app/controllers/jsonforem/errors_controller.rb:

require_dependency "jsonforem/application_controller"

module Jsonforem
  class ErrorsController < ApplicationController
    def not_found
      render :json => {:error => "Not found"}, :status => 404
    end
  end
end

What am I doing wrong in my exceptions_app configuration?