Add cfg test case for exceptions' __str__ methods
Add cfg test case for exceptions' __str__ methods

Increases coverage from 93% to 97%

Change-Id: I6a41b31e29238831fe2a888d5d64dc0bffd770c0

magic0704/oslo.config,JioCloud/oslo.config,citrix-openstack-build/oslo.config,metacloud/oslo.config,shad7/oslo.config,tianshan/oslo.config,varunarya10/oslo.config,citrix-openstack/build-oslo.config