[something]Continuation renderers, all of which are junk
except one. Check the items in each one until the one which
contains the items being sought is found.
The usage in extract_comments_info needed to be changed to
specify the items being sought. It was unspecified before which
is strictly incorrect since extract_items by default looks for
video/playlist/channel thumbnail items. It was relying on this
special case for continuations. But now that wouldn't work
anymore.
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
