code: func ReplaceNSEWith(err errors.MultiError, replacement error) error {
  	lme := errors.NewLazyMultiError(len(err))
  	for i, ierr := range err {
  		if ierr == datastore.ErrNoSuchEntity {
  			ierr = replacement
  		}
  		lme.Assign(i, ierr)
  	}
  	return lme.Get()
  }
docs: // ReplaceNSEWith takes an errors.MultiError returned by a datastore.Get() on a slice
  // (which is always a MultiError), filters out all datastore.ErrNoSuchEntitiy
  // or replaces it with replacement instances, and returns an error generated
  // by errors.LazyMultiError.