iSCSILogicalUnit: when deleting a LUN or initiator fails with lio-t, proceed with warning

During the stop operation, "targetcli delete" returns an error if the
object we're about to delete already no longer exists. Since stop is a
force operation and we must do anything we can to clean up the
resource, don't exit on that condition but log a warning and proceed
to removal of the backstore. If *that* fails, then we error out and
potentially cause the node to be fenced.

Fixes ClusterLabs/resource-agents#642.