extract_channel_info: Improve error extraction

Use extract_str function since it's not always 'simpleText'
Make sure we don't output an empty error message if we don't
know what it is.
channel.py: Don't check if error message is empty, check if it's
None

youtube/channel.py

@@ -203,7 +203,7 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
 
 
     info = yt_data_extract.extract_channel_info(json.loads(polymer_json), tab)
-    if info['error']:
+    if info['error'] is not None:
         return flask.render_template('error.html', error_message = info['error'])
 
     post_process_channel_info(info)