Correctly handle cassandra walk exeption

In case of excveption during the DB walk, the method
'config_object_error' could be call with obj_type set to None:
http://paste.ubuntu.com/8160393/

Change-Id: I8b4d9fa50d1f4ad55b1c77bb6ad6edcd82a9bdfe
Merge "Correctly handle cassandra walk exeption"