app-assets-html-404.html.erb

Whoops, 404!

assets.task

Rake::Task['assets:precompile'].enhance do
  assets_path = File.join(Rails.public_path, "assets")

  Dir[File.join(assets_path, '*.html*')].each do |matched_asset_path|
    http_status_code, extension = /#{assets_path}\/([0-9]{3})[^\/\s]*(\.html.?[^\b]*)/.match(matched_asset_path).captures
    error_page_path = File.join(Rails.public_path, "#{http_status_code}#{extension}")

    cp matched_asset_path, error_page_path
  end
end 

config-application.rb

config.assets.paths << "#{Rails.root}/assets/html"