[youku] better error handling

blocked videos used to cause death by TypeError, now we report what the
server says

youtube_dl/extractor/youku.py

@@ -66,6 +66,12 @@ class YoukuIE(InfoExtractor):
         self.report_extraction(video_id)
         try:
             config = json.loads(jsondata)
+            error_code = config['data'][0].get('error_code')
+            if error_code:
+                # -8 means blocked outside China.
+                error = config['data'][0].get('error')  # Chinese and English, separated by newline.
+                raise ExtractorError(error or u'Server reported error %i' % error_code,
+                    expected=True)
 
             video_title =  config['data'][0]['title']
             seed = config['data'][0]['seed']
@@ -89,6 +95,7 @@ class YoukuIE(InfoExtractor):
 
             fileid = config['data'][0]['streamfileids'][format]
             keys = [s['k'] for s in config['data'][0]['segs'][format]]
+            # segs is usually a dictionary, but an empty *list* if an error occured.
         except (UnicodeDecodeError, ValueError, KeyError):
             raise ExtractorError(u'Unable to extract info section')