Constant pg_async::pq::PG_DIAG_SQLSTATE

pub const PG_DIAG_SQLSTATE: c_int = _; // 67i32

According to https://www.postgresql.org/message-id/20041202060648.GA60984%40winnie.fuhr.org returns a error code from https://www.postgresql.org/docs/9.4/static/errcodes-appendix.html.