User defined EXCEPTIONs

Using postgreSQL 8.1 on fedora.

Below is a function RowCount_Select(Table_Name) defined that raises exception (test1) which i intend to catch in test1( ) and raise exception (test2).
Do we have a way to catch such user defined exceptions? Can someone direct me to the right resource pages?

create or replace function RowCount_Select(varchar) returns int as $$
DECLARE
    res int;
BEGIN
    select into res reltuples from pg_class where relkind='r' and relname=$1;
    if not found then
        raise exception 'test1';
    end if;
    return res;
END;
$$ language plpgsql strict;

-- There is no relation by name ' test '
create or replace function test1() returns int as $$
DECLARE
    res int;
BEGIN
    select into res RowCount_Select('test');
    return res;
EXCEPTION
    when test1 then
        raise exception 'test2';
END;
$$ language plpgsql strict;

ERROR:  unrecognized exception condition "test1"
CONTEXT:  compile of PL/pgSQL function "test1" near line 11

thanks,
vish