If somebody puts a window function in WHERE, we should complain about that
in so many words.  The previous coding tended to complain about the window
function's arguments instead, which is likely to be misleading to users who
are unclear on the semantics of window functions; as seen for example in
bug #6440 from Matyas Novak.

Just another example of how "add new code at the end" is frequently a bad
heuristic.