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/watch.html

@@ -172,7 +172,11 @@
                             {% else %}
                                 <li>{{ playlist['current_index']+1 }}/{{ playlist['video_count'] }}</li>
                             {% endif %}
+                            {% if playlist['author_url'] %}
                             <li><a href="{{ playlist['author_url'] }}" title="{{ playlist['author'] }}">{{ playlist['author'] }}</a></li>
+                            {% elif playlist['author'] %}
+                            <li>{{ playlist['author'] }}</li>
+                            {% endif %}
                         </ul>
                     </div>
                     <nav class="playlist-videos">