Alleviate #449 by catching bad metadata response

Instead of ungracefully failing with a TypeError, the Post is now skipped with
an error message containing the invalid JSON.

instaloader/structures.py

@@ -134,6 +134,12 @@ class Post:
             pic_json = self._context.get_json("p/{0}/".format(self.shortcode), params={})
             self._full_metadata_dict = pic_json['entry_data']['PostPage'][0]['graphql']['shortcode_media']
             self._rhx_gis_str = pic_json.get('rhx_gis')
+            if self._full_metadata_dict is None:
+                # issue #449
+                self._context.error("Fetching Post metadata failed (issue #449). "
+                                    "The following data has been returned:\n"
+                                    + json.dumps(pic_json['entry_data'], indent=2))
+                raise BadResponseException("Fetching Post metadata failed.")
             if self.shortcode != self._full_metadata_dict['shortcode']:
                 self._node.update(self._full_metadata_dict)
                 raise PostChangedException