hatpear.go
func Get(r *http.Request) error {
	errptr, ok := r.Context().Value(errorKey).(*error)
	if !ok {
		return nil
	}
	return *errptr
}
func Get(r *http.Request) error {
	errptr, ok := r.Context().Value(errorKey).(*error)
	if !ok {
		return nil
	}
	return *errptr
}
// Get retrieves an error from the request's context. It returns nil if the
// request was not configured to store errors.