code: def failure_format_traceback(self, fail):
          
          try:
              f = six.StringIO()
              traceback.print_exception(
                  fail._type,
                  fail.value,
                  fail._traceback,
                  file=f,
              )
              return f.getvalue()
          except Exception:
              return u"Failed to format failure traceback for '{0}'".format(fail)
docs: :param fail: must be an IFailedFuture
          returns a string