cmd/headscale/cli: silence cobra error/usage output and centralise error formatting

Set SilenceErrors and SilenceUsage on the root command so that
cobra never prints usage text for runtime errors. A SetFlagErrorFunc
callback re-enables usage output specifically for flag-parsing
errors (the kubectl pattern).

Add printError to utils.go and switch Execute() to ExecuteC() so
the returned error is formatted as JSON/YAML when --output requests
machine-readable output.

cmd/headscale/cli/utils.go

@@ -203,6 +203,34 @@ func ErrorOutput(errResult error, override string, outputFormat string) {
 	os.Exit(1)
 }
 
+// printError writes err to stderr, formatting it as JSON/YAML when the
+// --output flag requests machine-readable output.  Used exclusively by
+// Execute() so that every error surfaces in the format the caller asked for.
+func printError(err error, outputFormat string) {
+	type errOutput struct {
+		Error string `json:"error"`
+	}
+
+	e := errOutput{Error: err.Error()}
+
+	var formatted []byte
+
+	switch outputFormat {
+	case "json":
+		formatted, _ = json.MarshalIndent(e, "", "\t") //nolint:errchkjson // errOutput contains only a string field
+	case "json-line":
+		formatted, _ = json.Marshal(e) //nolint:errchkjson // errOutput contains only a string field
+	case "yaml":
+		formatted, _ = yaml.Marshal(e)
+	default:
+		fmt.Fprintf(os.Stderr, "Error: %s\n", err)
+
+		return
+	}
+
+	fmt.Fprintf(os.Stderr, "%s\n", formatted)
+}
+
 func HasMachineOutputFlag() bool {
 	for _, arg := range os.Args {
 		if arg == "json" || arg == "json-line" || arg == "yaml" {