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/watch.py

@@ -431,14 +431,20 @@ def extract_info(video_id, use_invidious, playlist_id=None, index=None):
     # Fallback to 'ios' if no valid URLs are found
     if not info.get('formats') or info.get('player_urls_missing'):
         print(f"No URLs found in '{primary_client}', attempting with '{fallback_client}'.")
-        player_response = fetch_player_response(fallback_client, video_id) or {}
-        yt_data_extract.update_with_new_urls(info, player_response)
+        try:
+            player_response = fetch_player_response(fallback_client, video_id) or {}
+            yt_data_extract.update_with_new_urls(info, player_response)
+        except util.FetchError as e:
+            print(f"Fallback '{fallback_client}' failed: {e}")
 
     # Final attempt with 'tv_embedded' if there are still no URLs
     if not info.get('formats') or info.get('player_urls_missing'):
         print(f"No URLs found in '{fallback_client}', attempting with '{last_resort_client}'")
-        player_response = fetch_player_response(last_resort_client, video_id) or {}
-        yt_data_extract.update_with_new_urls(info, player_response)
+        try:
+            player_response = fetch_player_response(last_resort_client, video_id) or {}
+            yt_data_extract.update_with_new_urls(info, player_response)
+        except util.FetchError as e:
+            print(f"Fallback '{last_resort_client}' failed: {e}")
 
     # signature decryption
     if info.get('formats'):