integration: return cli err when unmarshal fails

Signed-off-by: Kristoffer Dalby <[email protected]>

integration/cli_test.go

@@ -23,7 +23,7 @@ func executeAndUnmarshal[T any](headscale ControlServer, command []string, resul
 
 	err = json.Unmarshal([]byte(str), result)
 	if err != nil {
-		return err
+		return fmt.Errorf("failed to unmarshal: %s\n command err: %s", err, str)
 	}
 
 	return nil