Tweak plpgsql's expression reader to be smarter about parentheses and

Tweak plpgsql's expression reader to be smarter about parentheses and
to give more useful error messages.  Stephen Szabo's example of this
morning ('loop' used as a variable name inside a subselect) works
correctly now, and a FOR that is misinterpreted as an integer FOR will
draw 'missing .. at end of SQL expression', which is at least
marginally helpful.