[deviantart] handle decode errors for extended_fetch results (#655)

This isn't going to solve the underlying problem, but it should at
least provide the server response when those errors happen.

gallery_dl/extractor/deviantart.py

@@ -1044,7 +1044,10 @@ def deviation_extended_fetch(self, deviation_id, user, kind):
         if response.status_code == 404:
             raise exception.StopExtraction(
                 "Your account must use the Eclipse interface.")
-        return response.json()
+        try:
+            return response.json()
+        except Exception:
+            return {"error": response.text}
 
     def deviation_metadata(self, deviations):
         """ Fetch deviation metadata for a set of deviations"""