// for reporting at the end of the unmarshal.
func (d *decodeState) saveError(err error) {
	if d.savedError == nil {
		d.savedError = d.addErrorContext(err)
	}
}

// addErrorContext returns a new error enhanced with information from d.errorContext
func (d *decodeState) addErrorContext(err error) error {
	if d.errorContext != nil && (d.errorContext.Struct != nil || len(d.errorContext.FieldStack) > 0) {
		switch err := err.(type) {