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/comments.html

@@ -3,13 +3,13 @@
 {% macro render_comment(comment, include_avatar, timestamp_links=False) %}
     <div class="comment-container">
         <div class="comment">
-            <a class="author-avatar" href="{{ comment['author_url'] }}" title="{{ comment['author'] }}">
+            <a class="author-avatar" href="{{ comment['author_url'] or '#' }}" title="{{ comment['author'] }}">
                 {% if include_avatar %}
                     <img class="author-avatar-img" alt="{{ comment['author'] }}" src="{{ comment['author_avatar'] }}">
                 {% endif %}
             </a>
             <address class="author-name">
-                <a class="author" href="{{ comment['author_url'] }}" title="{{ comment['author'] }}">{{ comment['author'] }}</a>
+                <a class="author" href="{{ comment['author_url'] or '#' }}" title="{{ comment['author'] }}">{{ comment['author'] }}</a>
             </address>
             <a class="permalink" href="{{ comment['permalink'] }}" title="permalink">
                 <span>{{ comment['time_published'] }}</span>