Timestamped rack.errors log

app.rb

    class App < Sinatra::Base

      error_log = File.new('log/rack_errors.log', 'a+')
      error_log.sync = true

      def error_log.puts(*messages)
        messages.first.prepend "[#{Time.now.strftime '%F %X.%6N'}] "
        super
      end

      before do
        env["rack.errors"] = error_log
      end
    end