Improve error messages on UPDATE .. RETURNING (Subquery)

Currently we do not support having arbitrary subqueries in RETURNING clauses of an UPDATE query. If this subquery does not access the updated table, we see weird errors:

```
UPDATE ref SET a = 1 RETURNING
(SELECT pg_catalog.max(latest_end_time) FROM pg_catalog.pg_stat_wal_receiver)
as c3;
ERROR: a column definition list is required for functions returning "record"
```

similarly:

```
create table t1 (a int);
select create_reference_table('t1');
update t1 set a = 1 returning
(select pg_catalog.max(wal_bytes) from pg_catalog.pg_stat_wal)
as c3;
ERROR: input of anonymous composite types is not implemented
```

See: #6634