Remove custom errors page section from the guides

This pattern is too problematic and introduces a lot of edge cases:

  * On Rails 4.2, the issue https://github.com/rails/rails/issues/15124 is back again.
  * needs to define each action for each http status otherwise the router raises ActionController::RoutingError (No route matches).
  * If the router has `match "/*username",...` and some error page action is missing, Rails will pick up the "match" and try to do its job.
  * encourages people to copy & paste programming.

I've seen many people who have done this, and most of them have ran into an issue. I created [rambulance](https://github.com/yuki24/rambulance) gem to save them and it's the safest, easiest and simplest solution. I believe we should stop doing `config.exceptions_app = self.routes` and use static error pages, or use my gem as a temporary solution.

I would also love to merge rambulance gem into Rails itself if people like it.