def process_failure(result) #:nodoc:
      if @failure_callback
        @failure_callback.call(message, result)
      else
        failure(result)
      end
    rescue Exception
      Beetle::reraise_expectation_errors!
    end
def process_failure(result) #:nodoc:
      if @failure_callback
        @failure_callback.call(message, result)
      else
        failure(result)
      end
    rescue Exception
      Beetle::reraise_expectation_errors!
    end
should not be overriden in subclasses