feature request: include problematic values in UniqueViolationError

[good-idea]

Right now I'm using the .columns property on UniqueViolationErrors to format some human-readable error messages, i.e. This emailAddress is already in use. It would be great to be able to get the values that violate the constraint as well, to more easily get The emailAddress 'joseph@somewhere.com' is already in use.

Right now I can parse this out of the nativeError.detail property (I'm using PostgreSQL, not sure if this is a property specific to that).

Otherwise ~ thanks for this lib, it's super helpful to have a more structured way to deal with errors :)