Issue #3252533: Remove the ability for post-operation stage events to flag warnings

package_manager/src/Event/ErrorEventInterface.php

-<?php
-
-namespace Drupal\package_manager\Event;
-
-use Drupal\Core\StringTranslation\TranslatableMarkup;
-
-/**
- * Defines an interface for events which can collect validation errors.
- */
-interface ErrorEventInterface {
-
-  /**
-   * Adds a validation error.
-   *
-   * @param \Drupal\Core\StringTranslation\TranslatableMarkup[] $messages
-   *   The error messages.
-   * @param \Drupal\Core\StringTranslation\TranslatableMarkup|null $summary
-   *   (optional) The summary.
-   */
-  public function addError(array $messages, TranslatableMarkup $summary = NULL);
-
-}