func (iter *Iter) Err() error {
	iter.m.Lock()
	err := iter.err
	iter.m.Unlock()
	if err == ErrNotFound {
		return nil
	}
	return err
}
func (iter *Iter) Err() error {
	iter.m.Lock()
	err := iter.err
	iter.m.Unlock()
	if err == ErrNotFound {
		return nil
	}
	return err
}
// 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.