[Web][Dovecot] Do not count logins as failed while the identity provider is unavailable

data/web/inc/lib/mailcow/Auth/IdpUnavailable.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Mailcow\Auth;
+
+use RuntimeException;
+
+/**
+ * The identity provider could not answer a login (unreachable, timeout, server error). No failed login:
+ * callers report a temporary failure and must not count it as an attempt for netfilter.
+ */
+class IdpUnavailable extends RuntimeException
+{
+}