function exception

While writing a date validation function, I’m not sure which condition to check for in the following exception handler.

CREATE OR REPLACE FUNCTION IsValidDateTime(timestamp with time zone) RETURNS BOOLEAN AS $$

  BEGIN

    PERFORM CAST($1 AS TIMESTAMP WITH TIME ZONE);

    RETURN TRUE;

  EXCEPTION WHEN INVALID_DATETIME_FORMAT THEN

    RETURN FALSE;

  END

$$ LANGUAGE 'plpgsql';

If I call it this way:   SELECT IsValidDateTime(‘2008-05-06 00:00:95’);  

I would expect it to return FALSE but it returns:

ERROR:  date/time field value out of range: "2008-05-06 00:00:95"

Is there either a specific error code to use or is there a generic catch all code? I didn’t see anything better listed at http://www.postgresql.org/docs/8.0/static/errcodes-appendix.html

Thanks.