heat engine : avoid returning empty resource error strings
heat engine : avoid returning empty resource error strings

Avoid the possibility of returning an empty string (when Exceptions
have not got a string message), or parser.py will treat the error
as success and the resource operation failure will not be correctly
reflected in the resource state.

fixes bug 1130270

Change-Id: I1c55dea1f9615cd4b037802ff8c1066694bffea6

openstack/heat,NeCTAR-RC/heat,maestro-hybrid-cloud/heat,noironetworks/heat,jasondunsmore/heat,openstack/heat,NeCTAR-RC/heat,varunarya10/heat,cwolferh/heat-scratch,pshchelo/heat,dragorosson/heat,pratikmallya/heat,rh-s/heat,jasondunsmore/heat,pratikmallya/heat,Triv90/Heat,dims/heat,citrix-openstack-build/heat,takeshineshiro/heat,rdo-management/heat,dims/heat,citrix-openstack-build/heat,JioCloud/heat,pshchelo/heat,JioCloud/heat,Triv90/Heat,rh-s/heat,rickerc/heat_audit,takeshineshiro/heat,srznew/heat,ntt-sic/heat,gonzolino/heat,steveb/heat,redhat-openstack/heat,miguelgrinberg/heat,dragorosson/heat,redhat-openstack/heat,gonzolino/heat,maestro-hybrid-cloud/heat,Triv90/Heat,cryptickp/heat,miguelgrinberg/heat,srznew/heat,steveb/heat,noironetworks/heat,cryptickp/heat,ntt-sic/heat,cwolferh/heat-scratch,rickerc/heat_audit,rdo-management/heat,varunarya10/heat