Error message not correctly displayed

I have customized the capabilities, removing the possibility of creating guest users, but the editor is showing an error on displaying the error:

I'm aware of https://github.com/humanmade/authorship/issues/76, but this issue is about showing the right error message provided from the backend.

### How to reproduce:

1. Add in a plugin a code like this

```
add_filter('user_has_cap', 'removeGuestAuthorCap', 9999);

function removeGuestAuthorCap(array $user_caps): array
{
unset($user_caps['create_guest_authors']);

return $user_caps;
}
```
2. Login to WordPress admin panel as an user that's not a Super Admin
3. Try to add a guest author to a post
4. The error message shown in the editor should report "undefined"