Improve plpgsql's error messages for incorrect %TYPE and %ROWTYPE.

Improve plpgsql's error messages for incorrect %TYPE and %ROWTYPE.

If one of these constructs referenced a nonexistent object, we'd fall
through to feeding the whole construct to the core parser, which would
reject it with a "syntax error" message.  That's pretty unhelpful and
misleading.  There's no good reason for plpgsql_parse_wordtype and
friends not to throw a useful error for incorrect input, so make them
do that instead of returning NULL.

Discussion: https://postgr.es/m/1964516.1708977740@sss.pgh.pa.us