For improving WCAG compatibility we need OS to include a couple of additions when it generates an error message.
For a screenreader to pick up the warnings/exception messages the platform should include a ARIA role like: role="alert". I see no reason why these shouldn't always be included. ( also when de accessibility feature is disabled )
Additionally, it should be possible to tab to the message so that it can be dismissed; currently the keyboard-focussable is off, but it should be on.
We are now reverting to popups as these are requirements for a successful implementation of WCAG compatibility.