<?php

namespace Acme\HelloBundle\Controller;

use Guzzle\Http\Client;
use Guzzle\Http\Exception\BadResponseException;


public function someAction()
{
    try {
        // instantiate $form here ...
        $model = $form->getData(); // get the $model
        
        // convert the $model to a RESTfull $data and send it
        $client = new Client();
        $request = $client->post('someResource', null, json_encode($data)); // send the data to a RESTfull API
        $response = $request->send()->json();
        // ...
    } catch (ClientErrorResponseException $e) {
        if (Response::HTTP_BAD_REQUEST === $e->getResponse()->getStatusCode()) {
            $violations = json_decode($e->getResponse()->getBody(true), true);
    
            foreach($violations as $propertyPath => $violation) {
                $error = new FormError($this->get('translator')->trans($violation['message'], [], 'validators'), $violation['messageTemplate'], $violation['messageParameters'], $violation['messagePluralization']);
    
                $propertyPath = new PropertyPath($propertyPath);
    
                $field = null; // null or $form->get('aChild') if your property path is in a child
                foreach($propertyPath->getElements() as $element) {
                    $field = $field->get($element);
                }
                $field->addError($error);
            }
        }
    }
}