Add HandleResolutionException for failed handle resolution

src/Exceptions/HandleResolutionException.php

···
+<?php
+
+namespace SocialDept\AtpClient\Exceptions;
+
+class HandleResolutionException extends \Exception
+{
+    public function __construct(string $handle)
+    {
+        parent::__construct("Unable to resolve handle '{$handle}' to a DID");
+    }
+}