Rewrite ui.setPoster to check that images arent broken or youtube fallback images. Only show poster element when valid

src/js/plugins/youtube.js

@@ -162,7 +162,13 @@ const youtube = {
         player.media = utils.replaceElement(container, player.media);
 
         // Set poster image
-        player.media.setAttribute('poster', utils.format(player.config.urls.youtube.poster, videoId));
+        const posterSrc = format => `https://img.youtube.com/vi/${videoId}/${format}default.jpg`;
+
+        // Check thumbnail images in order of quality, but reject fallback thumbnails (120px wide)
+        utils.loadImage(posterSrc('maxres'), 121) // Higest quality and unpadded
+            .catch(() => utils.loadImage(posterSrc('sd'), 121)) // 480p padded 4:3
+            .catch(() => utils.loadImage(posterSrc('hq'))) // 360p padded 4:3. Always exists
+            .then(image => ui.setPoster.call(player, image.src));
 
         // Setup instance
         // https://developers.google.com/youtube/iframe_api_reference