Customizing user module output
Use this for errors:

/**
 * Implements hook_rest_server_execute_errors_alter().
 *
 * Reformat error output.
 */
function up_coaching_rest_rest_server_execute_errors_alter(&amp;$error_alter_array, $controller, $arguments) {
  $error_alter_array['body_data'] = array(
    'success' =&gt; FALSE,
    'error' =&gt; $error_alter_array['body_data'],
  );
}


For success output you need clone needed resources from user resource definition.