fix: translate page JavaScript doesn't load (hedyorg#1155)

The JavaScript app bundle depends on `client_messages.js` having
been loaded beforehand in the same page, but this script had not
yet been added to the HTML template.

Add an error message to indicate what went wrong, and fix the error.

static/js/auth.ts

@@ -33,6 +33,10 @@ interface UserForm {
   old_password?: string;
 }
 
+if (!(window as any).AuthMessages) {
+  throw new Error('On a page where you load this JavaScript, you must also load the "client_messages.js" script');
+}
+
 export const auth = {
   texts: AuthMessages,
   profile: undefined as (Profile | undefined),

templates/translate-fromto.html

@@ -72,6 +72,7 @@ <h2>{{file.caption}}</h2>
     </div>
 
     <script src="{{static('/vendor/jquery.min.js')}}" type="text/javascript" crossorigin="anonymous"></script>
+    <script src="/client_messages.js?lang={{ lang }}" type="text/javascript" crossorigin="anonymous"></script>
     <script src="{{static('/js/appbundle.js')}}" type="text/javascript" crossorigin="anonymous"></script>
   </body>
 </html>