[youtube:channel] Correct 404 handling

Previously, when we encountered a 404 - such as youtube-dl https://www.youtube.com/ohJeiboh8oorehai - we crashed with a regexp error. Instead, make sure to go on and eventually report a 404.

youtube_dl/extractor/youtube.py

@@ -1654,6 +1654,9 @@ class YoutubeChannelIE(InfoExtractor):
         channel_page = self._download_webpage(
             url + '?view=57', channel_id,
             'Downloading channel page', fatal=False)
+        if channel_page is False:
+            channel_playlist_id = False
+        else:
             channel_playlist_id = self._html_search_meta(
                 'channelId', channel_page, 'channel id', default=None)
             if not channel_playlist_id: