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/common_elements.html

@@ -20,7 +20,7 @@
             {{ info['error'] }}
         {% else %}
             <div class="item-video {{ info['type'] + '-item' }}">
-                <a class="thumbnail-box" href="{{ info['url'] }}" title="{{ info['title'] }}">
+                <a class="thumbnail-box" href="{{ info['url'] or '#' }}" title="{{ info['title'] }}">
                     <div class="thumbnail {% if info['type'] == 'channel' %} channel {% endif %}">
                         {% if lazy_load %}
                             <img class="thumbnail-img lazy" alt="&#x20;" data-src="{{ info['thumbnail'] }}" onerror="thumbnail_fallback(this)">
@@ -35,7 +35,7 @@
                         {% endif %}
                     </div>
                 </a>
-                <h4 class="title"><a href="{{ info['url'] }}" title="{{ info['title'] }}">{{ info['title'] }}</a></h4>
+                <h4 class="title"><a href="{{ info['url'] or '#' }}" title="{{ info['title'] }}">{{ info['title'] }}</a></h4>
 
                 {% if include_author %}
                     {% set author_description = info['author'] %}