Rendering error boxes are not accessible.

Rendering error boxes expand in details when clicked. There's no visual indication of such behaviour, and they are not keyboard focusable. We should improve the accessibility of those components.

This interactivity is done at : https://github.com/xwiki/xwiki-platform/blob/a5f837906cb6d6db8d28cfdcccd6b66b2cd30b4c/xwiki-platform-core/xwiki-platform-web/xwiki-platform-web-war/src/main/webapp/resources/js/xwiki/xwiki.js#L303-L319