[extractor/common] Wrap extractor errors (Fixes #1194)

For now, we just wrap some common errors. More may follow. We do not want to catch actual programming errors in the extractors, such as 1 // 0.

youtube_dl/extractor/common.py

@@ -264,8 +264,15 @@ def initialize(self):
 
     def extract(self, url):
         """Extracts URL information and returns it in list of dicts."""
-        self.initialize()
-        return self._real_extract(url)
+        try:
+            self.initialize()
+            return self._real_extract(url)
+        except ExtractorError:
+            raise
+        except compat_http_client.IncompleteRead as e:
+            raise ExtractorError('A network error has occured.', cause=e, expected=True)
+        except (KeyError,) as e:
+            raise ExtractorError('An extractor error has occured.', cause=e)
 
     def set_downloader(self, downloader):
         """Sets the downloader for this IE."""