Fix UniqueConstraintViolation for composite primary keys on sqlite

A unique constraint violation on a table with a composite primary key will currently raise `Sequel::DatabaseError: SQLite3::ConstraintException` instead of `Sequel::UniqueConstraintViolation`. The error message is slightly different (because english), so doesn't match the current regexp. Easy fix. Not sure if the guards are correct in the spec, I just copied/pasted from the one above, so please can you double check that.