only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\Core\Form\EventSubscriber;
 
+use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\ReplaceCommand;
 use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Form\FormAjaxException;
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
@@ -14,6 +16,7 @@
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
 use Symfony\Component\HttpKernel\KernelEvents;
 
 /**
@@ -64,9 +67,22 @@ public function onView(GetResponseForControllerResultEvent $event) {
    *   The event to process.
    */
   public function onException(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+
+    // @todo Comment.
+    if ($exception instanceof BadRequestHttpException) {
+      drupal_set_message($exception->getMessage(), 'error');
+      $response = new AjaxResponse();
+      $status_messages = array('#type' => 'status_messages');
+      $response->addCommand(new ReplaceCommand(NULL, \Drupal::service('renderer')->renderRoot($status_messages)));
+      $response->headers->set('X-Status-Code', 200);
+      $event->setResponse($response);
+      return;
+    }
+
     // Extract the form AJAX exception (it may have been passed to another
     // exception before reaching here).
-    if ($exception = $this->getFormAjaxException($event->getException())) {
+    if ($exception = $this->getFormAjaxException($exception)) {
       $request = $event->getRequest();
       $form = $exception->getForm();
       $form_state = $exception->getFormState();
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -22,6 +22,7 @@
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
 
 /**
  * Provides form building and processing.
@@ -264,6 +265,11 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
     // can use it to know or update information about the state of the form.
     $response = $this->processForm($form_id, $form, $form_state);
 
+    // @todo Comment.
+    if ($ajax_form_request && !isset($form_state->getUserInput()['form_id'])) {
+      throw new BadRequestHttpException(t('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports.', array('@size' => format_size(file_upload_max_size()))));
+    }
+
     // After processing the form, if this is an AJAX form request, interrupt
     // form rendering and return by throwing an exception that contains the
     // processed form and form state. This exception will be caught by