Safer federation of book data changes

Only broadcast to other BW instances, plus bonus error handling

bookwyrm/models/book.py

@@ -37,6 +37,10 @@ class BookDataModel(ObjectMixin, BookWyrmModel):
             self.remote_id = None
         return super().save(*args, **kwargs)
 
+    def broadcast(self, activity, sender, software='bookwyrm'):
+        ''' only send book data updates to other bookwyrm instances '''
+        super().broadcast(activity, sender, software=software)
+
 
 class Book(BookDataModel):
     ''' a generic book, which can mean either an edition or a work '''