Fix the handling of unavailable servers

See https://www.woltlab.com/community/thread/316319/

Author: dtdesign

wcfsetup/install/files/lib/system/package/PackageUpdateDispatcher.class.php

@@ -3,6 +3,7 @@
 namespace wcf\system\package;
 
 use GuzzleHttp\Exception\ClientException;
+use GuzzleHttp\Exception\ServerException;
 use GuzzleHttp\Psr7\Request;
 use GuzzleHttp\RequestOptions;
 use Psr\Http\Client\ClientExceptionInterface;
@@ -196,6 +197,10 @@ private function getPackageUpdateXML(PackageUpdateServer $updateServer)
                 $e->getResponse()->getBody(),
                 $updateServer,
             );
+        } catch (ServerException $e) {
+            throw new SystemException(
+                WCF::getLanguage()->get('wcf.acp.package.update.error.listNotFound') . ' (' . $e->getResponse()->getStatusCode() . ')'
+            );
         }
 
         if ($response->getStatusCode() !== 200 && $response->getStatusCode() !== 304) {