User-readable error messages

While trying to a delete a partner, one of our users encountered an error message

Integrity Error
null value in column "partner_id" violates not-null constraint
...and so on

There was already Invoice for that partner, so it could not be deleted.

I suggest there should be two kinds of error messages:
The user should be presented a meaningful, readable text message. Even if it's as undetailed as "Delete impossible because of linked transactions"
And then there could be a detailed one for the administrators, maybe via "details" button in the error message or in the logs.