Problem/Motivation
At least in some cases, it's possible to have a link to e.g. user/23 where the user had been deleted but at least for admins the link is still rendered.
Proposed resolution
This will mostly be prevented by:
#2350797: Move node delete cleanup from menu_ui to menu_link_content and handle all entities
However, we should also handle route/parameters not found more correctly and prevent the link from rendering except in the admin UI where it should be visually flagged.
Remaining tasks
doit
User interface changes
minor admin UI change
API changes
none