Arrange to give error when a SetOp member statement refers to a variable

of the containing query (which really can only happen in a rule context).
Per example from Brandon Craig Rhodes.  Also, make the error message
more specific for the similar case with sub-select in FROM.  The revised
coding should be easier to adapt to SQL99's LATERAL(), when we get around
to supporting that.

src/test/regress/expected/rangefuncs.out

@@ -18,7 +18,6 @@ INSERT INTO foo2 VALUES(1, 111);
 CREATE FUNCTION foot(int) returns setof foo2 as 'SELECT * FROM foo2 WHERE fooid = $1;' LANGUAGE SQL;
 -- supposed to fail with ERROR
 select * from foo2, foot(foo2.fooid) z where foo2.f2 = z.f2;
-NOTICE:  Adding missing FROM-clause entry for table "foo2"
 ERROR:  FROM function expression may not refer to other relations of same query level
 -- function in subselect
 select * from foo2 where f2 in (select f2 from foot(foo2.fooid) z where z.fooid = foo2.fooid) ORDER BY 1,2;