Code-Change & Basic custom error page

  1. Currently there's only a 404 page, but adding new ones should be
  easy.
  2. I wrote a model, a controller and a view in client side just for
  the error page.
  3. Ruby-side code for error page needs review. Some generated stuff
  may be removed.
  4. I'm not sure how to add 500 in ruby part, as there are multiple
  ruby controllers. Obviously, only adding exception handler for
  application_controller.rb won't be enough.