Respect\Validation\Exceptions\NestedValidationException::getMessages PHP Method

    public function getMessages()
    {
        $messages = [$this->getMessage()];
        foreach ($this as $exception) {
            $messages[] = $exception->getMessage();
        }
        if (count($messages) > 1) {
            array_shift($messages);
        }
        return $messages;
    }

 private function handleValidation(ServerRequestInterface $request, ResponseInterface $response, NestedValidationException $exception)
 {
     $body = json_encode(['message' => 'There was a problem when validating', 'errors' => $exception->getMessages()]);
     return $response->withStatus(400)->withHeader('Content-type', 'application/json')->write($body);
 }