Install psalm as dev, sync scripts updates

vendor/amphp/amp/lib/MultiReasonException.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Amp;
+
+class MultiReasonException extends \Exception
+{
+    /** @var \Throwable[] */
+    private $reasons;
+
+    /**
+     * @param \Throwable[] $reasons Array of exceptions rejecting the promise.
+     * @param string|null  $message
+     */
+    public function __construct(array $reasons, string $message = null)
+    {
+        parent::__construct($message ?: "Multiple errors encountered; use "
+            . self::class . "::getReasons() to retrieve the array of exceptions thrown");
+
+        $this->reasons = $reasons;
+    }
+
+    /**
+     * @return \Throwable[]
+     */
+    public function getReasons(): array
+    {
+        return $this->reasons;
+    }
+}