fix error handling, null URLs in templates, and Radio playlist support

- Global error handler: friendly messages for 429, 502, 403, 400
  instead of raw tracebacks. Filter FetchError from Flask logger.
- Fix None URLs in templates: protect href/src in common_elements,
  playlist, watch, and comments templates against None values.
- Radio playlists (RD...): redirect /playlist?list=RD... to
  /watch?v=...&list=RD... since YouTube only supports them in player.
- Wrap player client fallbacks (ios, tv_embedded) in try/catch so
  a failed fallback doesn't crash the whole page.

youtube/templates/playlist.html

@@ -10,11 +10,17 @@
 
     <div class="playlist-metadata">
         <div class="author">
+            {% if thumbnail %}
             <img alt="{{ title }}" src="{{ thumbnail }}">
+            {% endif %}
             <h2>{{ title }}</h2>
         </div>
         <div class="summary">
+            {% if author_url %}
             <a class="playlist-author" href="{{ author_url }}">{{ author }}</a>
+            {% else %}
+            <span class="playlist-author">{{ author }}</span>
+            {% endif %}
         </div>
         <div class="playlist-stats">
             <div>{{ video_count|commatize }} videos</div>