BlockForm is ignoring block plugin form errors.

The BlockForm is "embedding" the block plugin's configuration form under 'settings' key in the form() method via:
$form['settings'] = $entity->getPlugin()->buildConfigurationForm(array(), $form_state);

This is correct. But in the validateForm() method it completely ignores errors set by the block plugin. It will only take care of the values. The code is:

/**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);

    // The Block Entity form puts all block plugin form elements in the
    // settings form element, so just pass that to the block for validation.
    $settings = (new FormState())->setValues($form_state->getValue('settings'));
    // Call the plugin validate handler.
    $this->entity->getPlugin()->validateConfigurationForm($form, $settings);
    // Update the original form values.
    $form_state->setValue('settings', $settings->getValues());
    $this->validateVisibility($form, $form_state);
  }

whereas it should be:

/**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);

    // The Block Entity form puts all block plugin form elements in the
    // settings form element, so just pass that to the block for validation.
    $settings = (new FormState())->setValues($form_state->getValue('settings'));
    // Call the plugin validate handler.
    $this->entity->getPlugin()->validateConfigurationForm($form['settings'], $settings);
    // Update the original form values.
    $form_state->setValue('settings', $settings->getValues());
    $errors = $settings->getErrors();
    if (!empty($errors)) {
      foreach ($errors AS $name => $message) {
        $form_state->setErrorByName($name, $message);
      }
    }
    $this->validateVisibility($form, $form_state);
  }

So currently there is no way to properly in/validate settings form for block plugins. And I even think this bug is present in any implementation of this sort(ie. embedding forms within different forms, especially when it comes to plugins, this is not unusual).