[Validator] Improve message for unique entity constraint

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |no |
|New Feature? |no |
|BC Breaks?   |no |
|Deprecations?|no |
|Tests Pass?  |yes|
|Fixed Tickets|-  |
|License      |MIT|
|Doc PR       |-  |
                   


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the constraint is for an entity though, but i know you know that. So trying to understand how can we rephrase this better, I am just doing it from experience since i used this constraint to dedup entities.

Options are:

- Entity with provided fields becomes a duplicate.
- Fields passed not unique for this entity.
- Fields are not unique.
- ... ?