Add the following to your application.rb:

  protected  

  def log_error(exception) 
    super(exception)

    begin
      ErrorMailer.deliver_snapshot(
        exception, 
        clean_backtrace(exception), 
        @session.instance_variable_get("@data"), 
        @params, 
        @request.env)
    rescue => e
      logger.error(e)
    end
  end

Taken from http://wiki.rubyonrails.com/rails/show/HowtoSendEmailWhenRailsThrowsAnException