Example for not registered username in User namespace: https://en.wikipedia.org/wiki/User:UserNamaseasca
''User account "UserNamaseasca" is not registered.''
I propose same message in Special:Contributions.
The current page shows dialog for non-existing users as well, I've made an example how it can be done (see attachment). Optionally this text can be red.
Version: 1.23.0
Severity: enhancement
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=67203