Fixed a small bug, added error msg as suggested by cp16net
Fixed a small bug, added error msg as suggested by cp16net

mmasaki/trove,zhangg/trove,cp16net/trove,citrix-openstack-build/trove,redhat-openstack/trove,zhangg/trove,citrix-openstack/build-trove,citrix-openstack/build-trove,openstack/trove,cp16net/trove,denismakogon/trove-guestagent,citrix-openstack/build-trove,redhat-openstack/trove,changsimon/trove,zhujzhuo/openstack-trove,zhujzhuo/openstack-trove,denismakogon/trove-guestagent,hplustree/trove,fabian4/trove,cp16net/trove,redhat-openstack/trove,changsimon/trove,mmasaki/trove,citrix-openstack-build/trove,mmasaki/trove,hplustree/trove,citrix-openstack-build/trove,openstack/trove,fabian4/trove,zhujzhuo/openstack-trove,changsimon/trove,fabian4/trove