public Optional<String> getErrorMessage() {
    return error != null ? Optional.ofNullable(getRootCause(error).getMessage()) : Optional.empty();
  }
@Test
  public void getErrorMessage_returns_root_cause_message_if_error() {
    Exception rootCause = new IOException("fail to connect");
    Exception cause = new IOException("nested", rootCause);
    WebhookDelivery delivery = newBuilderTemplate()
      .setError(cause)
      .build();

    assertThat(delivery.getErrorMessage()).contains("fail to connect");
  }