FormErrorTrait.php

<?php

/**
     * @param $form
     * @return array
     */
    private function getErrorsAsArray($form)
    {
        $errors = [];
        foreach ($form->getErrors() as $error) {
            $params = $error->getMessageParameters();
            if (array_key_exists('{{ extra_fields }}', $params)) {
                $formattedParams = explode(', ', str_replace("\"", "", $params['{{ extra_fields }}']));
                $errors[]        = "This form should not contain extra fields: " . implode(', ', $formattedParams);
                continue;
            }

            $errors[] = $error->getMessage();
        }

        if ($form->hasChildren()) {
            foreach ($form->getChildren() as $child) {
                if (!$child->isValid()) {
                    array_merge($errors, $this->getErrorsAsArray($child);)
                }
            }
        }

        return $errors;
    }