test.rb

class NotOnTheListError < StandardError; end

module Security
  module Bouncer
    THE_LIST = %w(john disco_stu)

    def enter(name)
      unless THE_LIST.include?(name)
        raise NotOnTheListError.new("Sorry #{name}, you're not on the list")
      end
      puts "Hey #{name}, come on through."
      super(name)
    end
  end

  def self.prepended(base)
    class << base
      prepend Bouncer
    end
  end
end

module Club
  prepend Security

  def self.enter(name)
    puts "+#{name}"
  end
end

def club_simulation
  %w(john amy disco_stu).each do |name|
    begin
      Club.enter name
    rescue NotOnTheListError => e
      puts e.message
    end
  end
end

club_simulation