errors_engine.rb

class ErrorsEngine < Rails::Engine
  routes.draw do
    Site.all.each do |site|
      match '(errors)/:status', via: [:get, :post, :put, :patch, :delete], to: 'errors#show', constraints: { host: site.hostname, status: /\d{3}/ }
    end
  end
end

unless Rails.application.config.consider_all_requests_local
  Rails.application.config.exceptions_app = ErrorsEngine.routes
end

Rails.application.config.action_dispatch.rescue_responses.merge!(
  'Site::ConnectionError' => :service_unavailable,
  'Site::RecordNotFound'  => :not_found
)
ActionDispatch::ExceptionWrapper.rescue_responses.merge!(Rails.application.config.action_dispatch.rescue_responses)