[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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well, if you want something talking about your application domain, you will need to change it (this message is only the *default* message of the constraint).
But the current message is better than your suggestion IMO.