Default error page prevents an exception being mapped to the 500 error page

When there's a default error page configured, getErrorLocation(Throwable) will never return null so the logic added in UNDERTOW-249 never takes effect.

The logic that Undertow uses is:

1. See if the exception's been mapped
2. See if there's a default error page
3. See if 500's been mapped.

In contrast, Tomcat's logic is:

1. See if the exception's been mapped
2. See if 500's been mapped
3. See if there's a default error page

I've seen UNDERTOW-192 and have Undertow configured with ServletStackTraces.NONE.