Handles fallbacks for failure of fetch,
        wrapper for self._fetch

def _fetchOrFallback(self, _=None):
        """
        Handles fallbacks for failure of fetch,
        wrapper for self._fetch
        """
        res = yield self._fetch(None)
        if res == RC_SUCCESS:
            return res
        elif self.retryFetch:
            yield self._fetch(None)
        elif self.clobberOnFailure:
            yield self.clobber()
        else:
            raise buildstep.BuildStepFailed()