[proposal] Add an option for returning SQLSTATE in psql error message

Hi,

Currently on error psql is printing Postgres' PQerrorMessage text, but
there's no guarantee these messages are constant between Postgres
versions and it's a pain when using psql for writing regression tests,

Prior
  Disallow setting client_min_messages higher than ERROR.

a bad workaround was to discarded error, now you have to do something like
CREATE OR REPLACE FUNCTION catch_error(
    query text
)
    RETURNS void AS $$
DECLARE
BEGIN
        EXECUTE query;
        EXCEPTION WHEN OTHERS THEN
            RAISE 'Query failed: %', SQLSTATE;
END;
$$LANGUAGE plpgsql;

SELECT catch_error('foo');

What about a new \whatever for setting psql error to either PQerrorMessage or
PQresultErrorField(res, PG_DIAG_SQLSTATE) if available?