Simplify error return from 'ipset list'

exec.Output() already wraps the stderr output, so we don't need to do that.

npc/ipset/ipset.go

@@ -77,9 +77,9 @@ func (i *ipset) DestroyAll() error {
 
 // Fetch a list of all existing sets with a given prefix
 func (i *ipset) List(prefix string) ([]Name, error) {
-	output, err := exec.Command("ipset", "list", "-name", "-output", "plain").CombinedOutput()
+	output, err := exec.Command("ipset", "list", "-name", "-output", "plain").Output()
 	if err != nil {
-		return nil, errors.Wrapf(err, "ipset list failed: %s", output)
+		return nil, err
 	}
 
 	var selected []Name