[niconico] Catch deleted videos (closes #4064)

youtube_dl/extractor/niconico.py

@@ -12,6 +12,7 @@
     unified_strdate,
     parse_duration,
     int_or_none,
+    ExtractorError,
 )
 
 
@@ -108,6 +109,9 @@ def _real_extract(self, url):
                 flv_info_request, video_id,
                 note='Downloading flv info', errnote='Unable to download flv info')
 
+        if 'deleted=' in flv_info_webpage:
+            raise ExtractorError('The video has been deleted.',
+                expected=True)
         video_real_url = compat_urlparse.parse_qs(flv_info_webpage)['url'][0]
 
         # Start extracting information