code: def _fetchOrFallback(self, _=None):
          
          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()
docs: Handles fallbacks for failure of fetch,
          wrapper for self._fetch