plpgsql: different error message for root block

```sql
DROP PROCEDURE IF EXISTS p;
CREATE PROCEDURE p() LANGUAGE PLpgSQL AS $$
<>
BEGIN
<>
LOOP
continue block_label WHEN true;
END LOOP;
END;
$$;
CALL p();
```
```
-- postgres: ERROR: 42601: block label "block_label" cannot be used in CONTINUE
-- crdb: ERROR: there is no label "block_label" attached to any block or loop enclosing this statement
```
I think the difference is only present for the root block (as if the root block's label is ignored).

Jira issue: CRDB-37737