[youku] Report private videos (Closes #8498)

youtube_dl/extractor/youku.py

@@ -229,6 +229,9 @@ class YoukuIE(InfoExtractor):
             if error_note is not None and '因版权原因无法观看此视频' in error_note:
                 raise ExtractorError(
                     'Youku said: Sorry, this video is available in China only', expected=True)
+            elif error_note and '该视频被设为私密' in error_note:
+                raise ExtractorError(
+                    'Youku said: Sorry, this video is private', expected=True)
             else:
                 msg = 'Youku server reported error %i' % error.get('code')
                 if error_note is not None: