code: func (iter *Iter) Err() error {
  	iter.m.Lock()
  	err := iter.err
  	iter.m.Unlock()
  	if err == ErrNotFound {
  		return nil
  	}
  	return err
  }
docs: // Err returns nil if no errors happened during iteration, or the actual
  // error otherwise.
  //
  // In case a resulting document included a field named $err or errmsg, which are
  // standard ways for MongoDB to report an improper query, the returned value has
  // a *QueryError type, and includes the Err message and the Code.