Adding user to private workspace: give feedback when user already added/not registered on environment

Description of the limitation and why it is relevant to address

As an administrator of a workspace I want to get some form of feedback if I want to add a user that has already been added, or has not yet been registered on the VIKTOR environment so that it is clear to the administrator what the reason is, rather than thinking it is a bug.

I think this is relevant for the VIKTOR platform because it improves user experience.

Submitter proposed design (optional)


Current workarounds


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