I'm not too sure whether this is a bug or an enhancement but just in case I will report it:
when the war.xml file contains the form-login-page element but that page defined does not exist, here's the Undertow behavior:
1. the war file is successfully deployed
2. when you try to access to a secured resource, you will get the following error:
ERROR [io.undertow.request] (default task-7) UT005023: Exception handling request to /app/login.html: java.lang.IllegalStateException: UT010019: Response already commited

I'm not too sure whether the behavior should be redirecting to the 404 page not found page.

I know is a minor thing but just wanted to point it out in case is useful.