* __BREAKING:__ A `UniqueConstraintViolationError` may now contain different values, depending on whether the violated constraint was a composite across multiple columns or not. For single-column `UNIQUE` constraints, the `value` and `column` properties will still be present as before; however, for a composite constraint, those properties will be undefined and `values` and `columns` will be set instead.
* __Minor:__ Added support for detecting "undefined column" errors (as an `UndefinedColumnError`).