FormState::setErrorByName is inconsistent with other methods

This would be a nice change but it is not a bug, changing to a task. For fun, I updated the patch but it still needs work and I see there is a suggestion for a Unit test instead.

What's wrong with \Drupal\Core\Form\FormState::setError() - this implementation is

  /**
   * {@inheritdoc}
   */
  public function setError(array &$element, $message = '') {
    $this->setErrorByName(implode('][', $element['#parents']), $message);
    return $this;
  }

I think this should be closed as outdated.