Repost of Functions Returning Record Datatype

Hi,

I was wondering if I have a function that returns a record datatype, and I call it using the as keyword to specify the table structure, can I return a different structure than the one specified?

Say I call select * from xy ('abc', 'def') as (samplea char, sampleb char) for a function of

CREATE FUNCTION xy(char, char) RETURNS record AS'
Declare
testout record;
if userallowed = 0 then

else
for testout in ''select * from testtable'' loop
return next testout;
end loop;
end if;
return null;
end;'
language 'plpgsql';

Can I instead send back "errordescription text, errornumber int, errornext int"? How do I do that?

I have a web front end and I have to get custom database errors to the front end. Maybe there is something more simple?

TIA
Alex Erwin