Better error messages for %TYPE and %ROWTYPE in plpgsql

Per recent discussion[1], plpgsql returns fairly unhelpful "syntax
error" messages when a %TYPE or %ROWTYPE construct references a
nonexistent object. Here's a quick little finger exercise to try
to improve that.

The basic point is that plpgsql_parse_wordtype and friends are
designed to return NULL rather than failing (at least when it's
easy to do so), but that leaves the caller without enough info
to deliver a good error message. There is only one caller,
and it has no use at all for this behavior, so let's just
change those functions to throw appropriate errors. Amusingly,
plpgsql_parse_wordrowtype was already behaving that way, and
plpgsql_parse_cwordrowtype did so in more cases than not,
so we didn't even have a consistent "return NULL" story.

Along the way I got rid of plpgsql_parse_cwordtype's restriction
on what relkinds can be referenced. I don't really see the
point of that --- as long as the relation has the desired
column, the column's type is surely well-defined.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/88b574f4-cc08-46c5-826b-020849e5a356%40gelassene-pferde.biz