first commit

web/core/themes/classy/js/media_embed_ckeditor.theme.es6.js

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+/**
+ * @file
+ * Classy theme overrides for the Media Embed CKEditor plugin.
+ */
+
+(Drupal => {
+  /**
+   * Themes the error displayed when the media embed preview fails.
+   *
+   * @param {string} error
+   *   The error message to display
+   *
+   * @return {string}
+   *   A string representing a DOM fragment.
+   *
+   * @see media-embed-error.html.twig
+   */
+  Drupal.theme.mediaEmbedPreviewError = () =>
+    `<div class="media-embed-error media-embed-error--preview-error">${Drupal.t(
+      'An error occurred while trying to preview the media. Please save your work and reload this page.',
+    )}</div>`;
+})(Drupal);

web/core/themes/classy/js/media_embed_ckeditor.theme.js

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+/**
+* DO NOT EDIT THIS FILE.
+* See the following change record for more information,
+* https://www.drupal.org/node/2815083
+* @preserve
+**/
+
+(function (Drupal) {
+  Drupal.theme.mediaEmbedPreviewError = function () {
+    return '<div class="media-embed-error media-embed-error--preview-error">' + Drupal.t('An error occurred while trying to preview the media. Please save your work and reload this page.') + '</div>';
+  };
+})(Drupal);