Update access-om2 restart pruning error msg

Co-authored-by: Aidan Heerdegen <[email protected]>

payu/models/accessom2.py

@@ -93,4 +93,4 @@ def get_restart_datetime(self, restart_path):
                 mom_restart_path = os.path.join(restart_path, model.name)
                 return model.get_restart_datetime(mom_restart_path)
 
-        raise NotImplementedError('access-om2 date-based restart pruning currently only uses the mom sub-model to find restart dates')
+        raise NotImplementedError('Cannot find mom sub-model: access-om2 date-based restart pruning requires the mom sub-model to determine restart dates')