Improve `alt` and `title` attributes for missing entity indicator

Problem/Motivation

Until #2982322: Mark missing embedded entities in WYSIWYG landed, embeds of missing entities ( were deleted) would result in a broken situation for both the content author and the content consumer.

In #2982322, some UI strings were introduced.

Over at #2940029-40: Add an input filter to display embedded Media entities, Drupal core accessibility maintainer @andrewmacpherson pointed out some accessibility concerns:

+  protected function renderMissingMedia() {
+    return [
+      '#type' => 'html_tag',
+      '#tag' => 'img',
+      '#attributes' => [
+        'src' => file_url_transform_relative(file_create_url('core/modules/media/images/icons/no-thumbnail.png')),
+        'width' => 180,
+        'height' => 180,
+        'alt' => $this->t('Deleted content encountered, site owner alerted.'),

Is this text alternative going to be presented publicly to a website visitor? The message here describes the inner working of the CMS, and governance process in the organization who runs it. A public visitor doesn't benefit from being told about this. In any case, "site owner alerted" is not conveyed by the image itself; we don't want to provide extra information to screen reader users which we haven't provided for a sighted user. Also "deleted" isn't the same as "missing".

This is a tricky situation to come up with alt text for. I'd suggest something simpler, like "missing image" or "broken image". This would be a more matter-of-fact statement, which doesn't say anything about the CMS or owner's processes. It would be good to tailor it to the media type ("image" or "video" instead of vague "content").

Proposed resolution

Implement Andrew's suggestions, because that'll improve the experience for everyone.

Remaining tasks

User interface changes

Improved alt and title attributes.

API changes

None.

Data model changes

None.