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/playlist.py

@@ -78,6 +78,15 @@ def get_playlist_page():
         abort(400)
 
     playlist_id = request.args.get('list')
+
+    # Radio/Mix playlists (RD...) only work as watch page, not playlist page
+    if playlist_id.startswith('RD'):
+        first_video_id = playlist_id[2:]  # video ID after 'RD' prefix
+        return flask.redirect(
+            util.URL_ORIGIN + '/watch?v=' + first_video_id + '&list=' + playlist_id,
+            302
+        )
+
     page = request.args.get('page', '1')
 
     if page == '1':