[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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I however doubt this is indeed for the end user because it does not say anything about the application domain. This would actually need to change, or customized which symfony allows. So for that reason I am closing it. I proposed this change just because I think is more friendly towards the developer when writing tests for instance, but I think a better way is wrapping the constraint into a domain constraint...