Rename cause -> exception and make exception optional.
Rename cause -> exception and make exception optional.

jessicalucci/TaskManagement,pombredanne/taskflow-1,jimbobhickville/taskflow,openstack/taskflow,varunarya10/taskflow,jessicalucci/TaskManagement,openstack/taskflow,junneyang/taskflow,varunarya10/taskflow,citrix-openstack-build/taskflow,junneyang/taskflow,jimbobhickville/taskflow,citrix-openstack-build/taskflow,pombredanne/taskflow-1