Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/guzzlehttp/promises/src/RejectionException.php

@@ -0,0 +1,47 @@
+<?php
+namespace GuzzleHttp\Promise;
+
+/**
+ * A special exception that is thrown when waiting on a rejected promise.
+ *
+ * The reason value is available via the getReason() method.
+ */
+class RejectionException extends \RuntimeException
+{
+    /** @var mixed Rejection reason. */
+    private $reason;
+
+    /**
+     * @param mixed $reason       Rejection reason.
+     * @param string $description Optional description
+     */
+    public function __construct($reason, $description = null)
+    {
+        $this->reason = $reason;
+
+        $message = 'The promise was rejected';
+
+        if ($description) {
+            $message .= ' with reason: ' . $description;
+        } elseif (is_string($reason)
+            || (is_object($reason) && method_exists($reason, '__toString'))
+        ) {
+            $message .= ' with reason: ' . $this->reason;
+        } elseif ($reason instanceof \JsonSerializable) {
+            $message .= ' with reason: '
+                . json_encode($this->reason, JSON_PRETTY_PRINT);
+        }
+
+        parent::__construct($message);
+    }
+
+    /**
+     * Returns the rejection reason.
+     *
+     * @return mixed
+     */
+    public function getReason()
+    {
+        return $this->reason;
+    }
+}