// writeErrorResponse writes error headers
func writeErrorResponse(ctx context.Context, w http.ResponseWriter, err APIError, reqURL *url.URL) {
	switch err.HTTPStatusCode {
	case http.StatusServiceUnavailable, http.StatusTooManyRequests:
		// Set retry-after header to indicate user-agents to retry request after 60 seconds.
		// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After
		w.Header().Set(xhttp.RetryAfter, "60")
	}