Static error pages from dynamically generated assets.

README.md

The guide Asset Pipeline Error Pages now no longer works, since non-digest assets aren't generated in newer Rails apps. To fix this, I've replaced the config/initializers/exceptions.rb initializer that swaps out the middleware with the Rake task here, which will be run after Rails assets:precompile task.

assets.rake

namespace :assets do
  task :precompile do
    # copy generated HTML error pages into public path after asset generation
    %w(404.html 422.html 500.html).each do |file|
      asset_file = ActionController::Base.helpers.asset_digest_path(file)
      asset_path = Rails.public_path.join('assets', asset_file)
      public_path = Rails.public_path.join(file)
      cp asset_path, public_path
    end
  end
end

Very nice approach, too.
I've taken the base idea but used a different approach:

Rails 5+ has the ApplicationController.renderer, which can render any template at any time. This works better than the Asset Pipeline, because the view works almost exactly the same. I've build a small gem, that wraps that:

https://rubygems.org/gems/static_error_pages