Support undefined messages

app.php

@@ -1,5 +1,4 @@
 <?php
-
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
@@ -66,7 +65,8 @@ header("Link: <static/js/bootstrap.bundle.min.js>; rel=preload; as=script", fals
 
         <?php
 // Alert messages
-        if (isset($_GET['msg']) && !is_empty($_GET['msg']) && array_key_exists($_GET['msg'], MESSAGES)) {
+        if (!empty($_GET['msg'])) {
+            if (array_key_exists($_GET['msg'], MESSAGES)) {
                 // optional string generation argument
                 if (!isset($_GET['arg']) || is_empty($_GET['arg'])) {
                     $alertmsg = $Strings->get(MESSAGES[$_GET['msg']]['string'], false);
@@ -89,6 +89,12 @@ header("Link: <static/js/bootstrap.bundle.min.js>; rel=preload; as=script", fals
                         $alerticon = "check";
                         break;
                 }
+            } else {
+                // We don't have a message for this, so just assume an error and escape stuff.
+                $alertmsg = htmlentities($Strings->get($_GET['msg'], false));
+                $alerticon = "times";
+                $alerttype = "danger";
+            }
             echo <<<END
             <div class="row justify-content-center" id="msg-alert-box">
                 <div class="col-11 col-sm-6 col-md-5 col-lg-4 col-xl-4">