Show Warning When a markdown.styles fails to load (#27105)

Fixes #8488

**Bug**
Currently there is no indication in the markdown preview when a `markdown.styles` element fails to load

**Fix**
Show an alert then a stylesheet does not load

extensions/markdown/media/loading.js

@@ -0,0 +1,26 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+(function () {
+	const unloadedStyles = [];
+
+	window.onStyleLoadError = (event) => {
+		const source = event.target.dataset.source;
+		unloadedStyles.push(source);
+	};
+
+	window.addEventListener('load', () => {
+		if (!unloadedStyles.length) {
+			return;
+		}
+		const args = [unloadedStyles];
+		window.parent.postMessage({
+			command: 'did-click-link',
+			data: `command:_markdown.onPreviewStyleLoadError?${encodeURIComponent(JSON.stringify(args))}`
+		}, 'file://');
+	});
+}());