Errors explains how to enable custom errors in Rails.

Errors.md

1. Register your application by appending this to config/application.rb:

   config.exceptions_app = self.routes
   

2. Add routes by appending this to config/routes.rb:

   match '/404', :to => 'errors#not_found'
   match '/422', :to => 'errors#server_error'
   match '/500', :to => 'errors#server_error'
   

3. Add an ErrorsController and views by running:

   rails g controller Errors not_found server_error
   

4. View custom error pages locally by setting this in config/development.rb

   config.consider_all_requests_local = false