code: func isInternalDSError(err error) bool {
  	if err == nil {
  		return false
  	}
  
  	merr, _ := err.(errors.MultiError)
  	if merr == nil {
  		return true // an overall RPC error
  	}
  
  	for _, e := range merr {
  		if e != nil && e != datastore.ErrNoSuchEntity {
  			return true // some serious issues with this single entity
  		}
  	}
  
  	return false // all suberrors are either nil or ErrNoSuchEntity
  }
docs: // isInternalDSError is true for datastore errors that aren't entirely
  // ErrNoSuchEntity.