code: def failure_message(self, fail):
          
          try:
              return u'{0}: {1}'.format(
                  fail._value.__class__.__name__,
                  str(fail._value),
              )
          except Exception:
              return u'Failed to produce failure message for "{0}"'.format(fail)
docs: :param fail: must be an IFailedFuture
          returns a unicode error-message