[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       |-  |
                   


 Sent using [Gush](https://github.com/gushphp/gush)
"This value is already used" is both consistent with other validation error messages and clear. I don't see why we would need to improve this one?