<?php
// put this in your controller

protected function getErrorsFromForm(FormInterface $form)
{
    $errors = array();
    foreach ($form->getErrors() as $error) {
        $errors[] = $error->getMessage();
    }

    foreach ($form->all() as $childForm) {
        if ($childForm instanceof FormInterface) {
            if ($childErrors = $this->getErrorsFromForm($childForm)) {
                $errors[$childForm->getName()] = $childErrors;
            }
        }
    }

    return $errors;
}