[kuwo] treat the offline error as an expected ExtractorError

youtube_dl/extractor/kuwo.py

@@ -78,7 +78,7 @@ class KuwoIE(KuwoBaseIE):
             url, song_id, note='Download song detail info',
             errnote='Unable to get song detail info')
         if '对不起，该歌曲由于版权问题已被下线，将返回网站首页' in webpage:
-            raise ExtractorError('this song has been offline because of copyright issues')
+            raise ExtractorError('this song has been offline because of copyright issues', expected=True)
 
         song_name = self._html_search_regex(
             r'(?s)class="(?:[^"\s]+\s+)*title(?:\s+[^"\s]+)*".*?<h1[^>]+title="([^"]+)"', webpage, 'song name')