fleetctl:destroy: on destroy check if the unit does exist or not

Do not error out directly if Destroy command fails, check first if the
unit does really exist if no then ignore the destroy error and continue.

Follow-up fix for: coreos#1383

fleetctl/destroy.go

@@ -42,6 +42,11 @@ func runDestroyUnits(args []string) (exit int) {
 	for _, v := range units {
 		err := cAPI.DestroyUnit(v.Name)
 		if err != nil {
+			// If unit does not exist do not error out
+			u, _ := cAPI.Unit(v.Name)
+			if u == nil {
+				continue
+			}
 			stderr("Error destroying units: %v", err)
 			exit = 1
 			continue