add temporary message for lack of ISBN service (see #276)

pubs/commands/add_cmd.py

@@ -78,9 +78,12 @@ def bibentry_from_api(args, ui, raw=False):
         if args.doi is not None:
             return apis.get_bibentry_from_api(args.doi, 'doi', ui=ui, raw=raw)
         elif args.isbn is not None:
+            ui.error('adding with ISBN relied on the OttoBib (https://www.ottobib.com/) service, that is no longer available. '
+                     'While we work on a new solution, you can manually generate bibtex from https://zbib.org/. '
+                     'If you know a good scriptable website/API, you tell us at https://github.com/pubs/pubs/issues/276.')
+            ui.exit(1)
             return apis.get_bibentry_from_api(args.isbn, 'isbn', ui=ui, raw=raw)
             # TODO distinguish between cases, offer to open the error page in a webbrowser.
-            # TODO offer to confirm/change citekey
         elif args.arxiv is not None:
             return apis.get_bibentry_from_api(args.arxiv, 'arxiv', ui=ui, raw=raw)
     except apis.ReferenceNotFoundError as e: