code: def errback(self, failure):
          
          self._setResult(failure)
          self._isFailure = True
  
          for d in self._deferreds:
              d.errback(failure)
docs: Errbacks the deferreds previously produced by this object.
  
          @param failure: The object which will be passed to the
          C{errback} method of all C{Deferred}s previously produced by
          this object's C{tee} method.
          @raise AlreadyCalledError: If L{callback} or L{errback} has
          already been called on this object.