Improve plpgsql parsing to report "foo is not a known variable", rather than a

Improve plpgsql parsing to report "foo is not a known variable", rather than a
generic syntax error, when seeing "foo := something" and foo isn't recognized.
This buys back most of the helpfulness discarded in my previous patch by not
throwing errors when a qualified name appears to match a row variable but the
last component doesn't match any field of the row.  It covers other cases
where our error messages left something to be desired, too.