decode.go
func (d *StreamDecoder) Err() error {
	if d.err == End {
		return nil
	}
	return d.err
}
func (d *StreamDecoder) Err() error {
	if d.err == End {
		return nil
	}
	return d.err
}
// Err returns the last error returned by the Decode method.
//
// The method returns nil if the stream reached its natural end.