diff --git a/core/modules/image/src/Controller/ImageStyleDownloadController.php b/core/modules/image/src/Controller/ImageStyleDownloadController.php
index fb6d96a..afeb20d 100644
--- a/core/modules/image/src/Controller/ImageStyleDownloadController.php
+++ b/core/modules/image/src/Controller/ImageStyleDownloadController.php
@@ -4,6 +4,7 @@
 
 use Drupal\Component\Utility\Crypt;
 use Drupal\Core\Image\ImageFactory;
+use Drupal\Core\Image\ImageInterface;
 use Drupal\Core\Lock\LockBackendInterface;
 use Drupal\image\ImageStyleInterface;
 use Drupal\system\FileDownloadController;
@@ -89,7 +90,7 @@ public function deliver(Request $request, $scheme, ImageStyleInterface $image_st
     $target = $request->query->get('file');
     $image_uri = $scheme . '://' . $target;
 
-    $this->validateRequest($request, $scheme, $image_style, $target, $image_uri);
+    $this->validateRequest($request, $image_style, $scheme, $target);
 
     $derivative_uri = $image_style->buildUri($image_uri);
     $headers = array();
@@ -117,27 +118,35 @@ public function deliver(Request $request, $scheme, ImageStyleInterface $image_st
       return new Response($this->t('Error generating image, missing source file.'), 404);
     }
 
-    $success = $this->generate($image_style, $image_uri, $derivative_uri);
-
-    if ($success) {
-      return $this->send($scheme, $derivative_uri, $headers);
+    try {
+      $image = $this->generate($image_style, $image_uri, $derivative_uri);
+      return $this->send($scheme, $image, $headers);
     }
-    else {
+    catch (\RuntimeException $e) {
       $this->logger->notice('Unable to generate the derived image located at %path.', array('%path' => $derivative_uri));
       return new Response($this->t('Error generating image.'), 500);
     }
   }
 
   /**
-   * @param $image_uri
+   * Validate that a source image exists, checking for double extensions.
+   *
+   * If the image style converted the extension, it has been added to the
+   * original file, resulting in filenames like image.png.jpeg. So to find
+   * the actual source image, we remove the extension and check if that
+   * image exists.
+   *
+   * @param string $image_uri
+   *   The URI to the source image.
+   *
    * @return string
+   *   The original $image_uri, or the source with the original extension.
+   *
+   * @throws \Symfony\Component\HttpFoundation\File\Exception\FileNotFoundException
+   *   Thrown when no valid source image is found.
    */
   protected function validateSource($image_uri) {
     if (!file_exists($image_uri)) {
-      // If the image style converted the extension, it has been added to the
-      // original file, resulting in filenames like image.png.jpeg. So to find
-      // the actual source image, we remove the extension and check if that
-      // image exists.
       $path_info = pathinfo($image_uri);
       $converted_image_uri = $path_info['dirname'] . DIRECTORY_SEPARATOR . $path_info['filename'];
       if (!file_exists($converted_image_uri)) {
@@ -151,13 +160,19 @@ protected function validateSource($image_uri) {
   }
 
   /**
-   * @param $scheme
-   * @param $derivative_uri
-   * @param $headers
+   * Return a response of the derived image.
+   *
+   * @param string $scheme
+   *   The URI scheme of $derivative_uri.
+   * @param \Drupal\core\Image\ImageInterface $image
+   *   The URI of the derived image.
+   * @param array $headers
+   *   (optional) An array of headers to return in the response.
+   *
    * @return \Symfony\Component\HttpFoundation\BinaryFileResponse
+   *   A response with the derived image.
    */
-  protected function send($scheme, $derivative_uri, $headers) {
-    $image = $this->imageFactory->get($derivative_uri);
+  protected function send($scheme, ImageInterface $image, $headers = array()) {
     $uri = $image->getSource();
     $headers += array(
       'Content-Type' => $image->getMimeType(),
@@ -171,10 +186,17 @@ protected function send($scheme, $derivative_uri, $headers) {
   }
 
   /**
+   * Generate an image derivative.
+   *
    * @param \Drupal\image\ImageStyleInterface $image_style
-   * @param $image_uri
-   * @param $derivative_uri
-   * @return bool
+   *   The image style to use for the derivative.
+   * @param string $image_uri
+   *   The URI of the original image.
+   * @param string $derivative_uri
+   *   The URI of the derived image.
+   *
+   * @return \Drupal\core\Image\ImageInterface
+   *   The generated image derivative.
    */
   protected function generate(ImageStyleInterface $image_style, $image_uri, $derivative_uri) {
     // Don't start generating the image if the derivative already exists or if
@@ -192,34 +214,47 @@ protected function generate(ImageStyleInterface $image_style, $image_uri, $deriv
     // Try to generate the image, unless another thread just did it while we
     // were acquiring the lock.
     $success = file_exists($derivative_uri) || $image_style->createDerivative($image_uri, $derivative_uri);
-
     if (!empty($lock_acquired)) {
       $this->lock->release($lock_name);
     }
 
-    return $success;
+    if (!$success) {
+      throw new \RuntimeException(sprintf('%s was unable to be generated', $derivative_uri));
+    }
+
+    return $this->imageFactory->get($derivative_uri);
   }
 
   /**
+   * Validate an incoming derivative request.
+   *
+   * Check that the style is defined, the scheme is valid, and the image
+   * derivative token is valid. Sites which require image derivatives to be
+   * generated without a token can set the
+   * 'image.settings:allow_insecure_derivatives' configuration to TRUE to
+   * bypass the latter check, but this will increase the site's vulnerability
+   * to denial-of-service attacks. To prevent this variable from leaving the
+   * site vulnerable to the most serious attacks, a token is always required
+   * when a derivative of a style is requested.
+   * The $target variable for a derivative of a style has
+   * styles/<style_name>/... as structure, so we check if the $target variable
+   * starts with styles/.
+   *
    * @param \Symfony\Component\HttpFoundation\Request $request
-   * @param $scheme
+   *   The incoming derivative request.
    * @param \Drupal\image\ImageStyleInterface $image_style
-   * @param $target
-   * @param $image_uri
+   *   The image style to use for the derivative.
+   * @param string $scheme
+   *   The URI scheme of $target.
+   * @param string $target
+   *   The path for the generated derivative.
+   *
+   * @throws \Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException
+   *   Thrown when the image style, the scheme, or the path token is invalid.
    */
-  protected function validateRequest(Request $request, $scheme, ImageStyleInterface $image_style, $target, $image_uri) {
-    // Check that the style is defined, the scheme is valid, and the image
-    // derivative token is valid. Sites which require image derivatives to be
-    // generated without a token can set the
-    // 'image.settings:allow_insecure_derivatives' configuration to TRUE to
-    // bypass the latter check, but this will increase the site's vulnerability
-    // to denial-of-service attacks. To prevent this variable from leaving the
-    // site vulnerable to the most serious attacks, a token is always required
-    // when a derivative of a style is requested.
-    // The $target variable for a derivative of a style has
-    // styles/<style_name>/... as structure, so we check if the $target variable
-    // starts with styles/.
+  protected function validateRequest(Request $request, ImageStyleInterface $image_style, $scheme, $target) {
     $valid = !empty($image_style) && file_stream_wrapper_valid_scheme($scheme);
+    $image_uri = $scheme . '://' . $target;
     if (!$this->config('image.settings')
         ->get('allow_insecure_derivatives') || strpos(ltrim($target, '\/'), 'styles/') === 0
     ) {