AD Auth: better auth failure (librenms#18471)

Throw an AuthenticationException if we cannot search for a user (permission removed or no bind user when enabling remember me)

Author: murrant

LibreNMS/Authentication/ActiveDirectoryAuthorizer.php

@@ -118,6 +118,11 @@ public function userExists($username, $throw_exception = false)
             $this->userFilter($username),
             ['samaccountname']
         );
+
+        if ($search === false) {
+            throw new AuthenticationException('User search failed: ' . ldap_error($connection));
+        }
+
         $entries = ldap_get_entries($connection, $search);
 
         if ($entries['count']) {