ErrorBag.php

<?php

namespace App\Form;
use JsonSerializable;

class ErrorBag implements JsonSerializable
{

    protected $_errors;

    public function __construct($errors)
    {
        $this->_errors = $errors;
    }

    public function jsonSerialize()
    {
        $result = [];
        foreach ($this->_errors as $field => $error) {
            $result[] = [
                'code' => key($error),
                'parameter' => $field,
                'detail' => current($error)
            ];
        }

        return $result;
    }
}

MyController.php

<?php

$entityWithErrors = ...;
$errors = new ErrorBag($entityWithErrors->errors());
$this->set('errors', $errors);
$this->set('_serialize', ['errors']);