plpgsql: error for txn control inside exception-handling block is eager

This is a tracking issue for a difference in behavior between CRDB and postgres. Postgres does not allow COMMIT or ROLLBACK statements inside a PL/pgSQL block that has an exception handler. However, postgres checks this condition lazily, so that the error is only thrown if the transaction control statement is actually executed:
```
CREATE PROCEDURE p(a INT) LANGUAGE PLpgSQL AS $$
BEGIN
IF a > 0 THEN
COMMIT;
END IF;
EXCEPTION WHEN division_by_zero THEN
RAISE NOTICE 'foo';
END;
$$;

CALL p(0); -- Postgres does not raise an error.
CALL p(1); -- Postgres raises an error.
```
CRDB eagerly checks for this situation, and throws the error in response to the `CREATE PROCEDURE` statement instead. This behavior is unlikely to change because users are unlikely to want to write such a stored procedure.

Jira issue: CRDB-36295