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/__init__.py

@@ -5,6 +5,7 @@ from flask import request
 import jinja2
 import settings
 import traceback
+import logging
 import re
 from sys import exc_info
 from flask_babel import Babel
@@ -12,6 +13,15 @@ from flask_babel import Babel
 yt_app = flask.Flask(__name__)
 yt_app.config['TEMPLATES_AUTO_RELOAD'] = True
 yt_app.url_map.strict_slashes = False
+
+# Don't log full tracebacks for handled FetchErrors
+class FetchErrorFilter(logging.Filter):
+    def filter(self, record):
+        if record.exc_info and record.exc_info[0] == util.FetchError:
+            return False
+        return True
+
+yt_app.logger.addFilter(FetchErrorFilter())
 # yt_app.jinja_env.trim_blocks = True
 # yt_app.jinja_env.lstrip_blocks = True
 
@@ -124,49 +134,54 @@ def timestamps(text):
 @yt_app.errorhandler(500)
 def error_page(e):
     slim = request.args.get('slim', False) # whether it was an ajax request
-    if (exc_info()[0] == util.FetchError
-        and exc_info()[1].code == '429'
-        and settings.route_tor
-    ):
-        error_message = ('Error: YouTube blocked the request because the Tor'
-            ' exit node is overutilized. Try getting a new exit node by'
-            ' using the New Identity button in the Tor Browser.')
-        if exc_info()[1].error_message:
-            error_message += '\n\n' + exc_info()[1].error_message
-        if exc_info()[1].ip:
-            error_message += '\n\nExit node IP address: ' + exc_info()[1].ip
-        return flask.render_template('error.html', error_message=error_message, slim=slim), 502
-    elif exc_info()[0] == util.FetchError and exc_info()[1].error_message:
-        # Handle specific error codes with user-friendly messages
-        error_code = exc_info()[1].code
-        error_msg = exc_info()[1].error_message
+    if exc_info()[0] == util.FetchError:
+        fetch_err = exc_info()[1]
+        error_code = fetch_err.code
+
+        if error_code == '429' and settings.route_tor:
+            error_message = ('Error: YouTube blocked the request because the Tor'
+                ' exit node is overutilized. Try getting a new exit node by'
+                ' using the New Identity button in the Tor Browser.')
+            if fetch_err.error_message:
+                error_message += '\n\n' + fetch_err.error_message
+            if fetch_err.ip:
+                error_message += '\n\nExit node IP address: ' + fetch_err.ip
+            return flask.render_template('error.html', error_message=error_message, slim=slim), 502
+
+        elif error_code == '429':
+            error_message = ('YouTube is temporarily blocking requests from your IP address (429 Too Many Requests).\n\n'
+                'Try:\n'
+                '• Wait a few minutes and refresh\n'
+                '• Enable Tor routing in Settings for automatic IP rotation\n'
+                '• Use a VPN to change your IP address')
+            if fetch_err.ip:
+                error_message += '\n\nYour IP: ' + fetch_err.ip
+            return flask.render_template('error.html', error_message=error_message, slim=slim), 429
+
+        elif error_code == '502' and ('Failed to resolve' in str(fetch_err) or 'Failed to establish' in str(fetch_err)):
+            error_message = ('Could not connect to YouTube.\n\n'
+                'Check your internet connection and try again.')
+            return flask.render_template('error.html', error_message=error_message, slim=slim), 502
+
+        elif error_code == '403':
+            error_message = ('YouTube blocked this request (403 Forbidden).\n\n'
+                'Try enabling Tor routing in Settings.')
+            return flask.render_template('error.html', error_message=error_message, slim=slim), 403
 
-        if error_code == '400':
-            error_message = (f'Error: Bad Request (400)\n\n{error_msg}\n\n'
-                           'This usually means the URL or parameters are invalid. '
-                           'Try going back and trying a different option.')
         elif error_code == '404':
             error_message = 'Error: The page you are looking for isn\'t here.'
-        else:
-            error_message = f'Error: {error_code} - {error_msg}'
+            return flask.render_template('error.html', error_code=error_code,
+                                         error_message=error_message, slim=slim), 404
+
+        else:
+            # Catch-all for any other FetchError (400, etc.)
+            error_message = f'Error communicating with YouTube ({error_code}).'
+            if fetch_err.error_message:
+                error_message += '\n\n' + fetch_err.error_message
+            return flask.render_template('error.html', error_message=error_message, slim=slim), 502
 
-        return (flask.render_template(
-                    'error.html',
-                    error_message=error_message,
-                    slim=slim
-                ), 502)
-    elif (exc_info()[0] == util.FetchError
-        and exc_info()[1].code == '404'
-    ):
-        error_message = ('Error: The page you are looking for isn\'t here.')
-        return flask.render_template('error.html',
-                                     error_code=exc_info()[1].code,
-                                     error_message=error_message,
-                                     slim=slim), 404
     return flask.render_template('error.html', traceback=traceback.format_exc(),
-                                 error_code=exc_info()[1].code,
                                  slim=slim), 500
-    # return flask.render_template('error.html', traceback=traceback.format_exc(), slim=slim), 500
 
 
 font_choices = {