When bulk-deleting, skip objects which are not "deletable"

whatever that means in context

tailbone/views/master.py

@@ -1728,7 +1728,8 @@ class MasterView(View):
     def bulk_delete_objects(self, session, objects, progress=None):
 
         def delete(obj, i):
-            self.delete_instance(obj)
+            if self.deletable_instance(obj):
+                self.delete_instance(obj)
             if i % 1000 == 0:
                 session.flush()