Edit Category Screen: Improve error message when trying to update a category with an existing slug

[plari]

Request: improve the error message shown on the Category Edit screen when trying to set an already-used slug.

Why:

If I edit a category and try to give it a slug that is already used on another category, I encounter the vague error Category not updated.

This error is not very informative.

By comparison, if I Quick-Edit to make this change, though, I am told: The slug “example-slug” is already in use by another term., which is much clearer and I understand the problem right away.

[plari]

Recording of the different error messages