Arrange for ORDER BY an expression on a UNION/INTERSECT/EXCEPT result,

such as
    SELECT f1 FROM foo UNION SELECT ... ORDER BY upper(f1)
to draw
'ORDER BY on a UNION/INTERSECT/EXCEPT result must be on one of the result columns'
rather than the uninformative 'f1 not found' we were producing before.
Eventually this should actually work, but that looks much too hard to try
to implement in late beta...