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

Add IsErrorUnitNotFound() and use it in destroy to check if the error
indicates that the unit does not exist.

For the moment we just want to check if the error is 'unit not found', if
later we want more we may export a more generic function into cAPI.

client/http.go

@@ -137,3 +137,7 @@ func is404(err error) bool {
 	googerr, ok := err.(*googleapi.Error)
 	return ok && googerr.Code == http.StatusNotFound
 }
+
+func IsErrorUnitNotFound(err error) bool {
+	return is404(err)
+}

fleetctl/destroy.go

@@ -16,6 +16,8 @@ package main
 
 import (
 	"time"
+
+	"github.com/coreos/fleet/client"
 )
 
 var cmdDestroyUnit = &Command{
@@ -42,6 +44,10 @@ func runDestroyUnits(args []string) (exit int) {
 	for _, v := range units {
 		err := cAPI.DestroyUnit(v.Name)
 		if err != nil {
+			// Ignore 'Unit does not exist' error
+			if client.IsErrorUnitNotFound(err) {
+				continue
+			}
 			stderr("Error destroying units: %v", err)
 			exit = 1
 			continue