In FirebirdSql, how to return exception message from procedure

I want to return the error message from a procedure when an exception happens. In SQL Server you would select the Error_Number() and Error_Message(). How would I do it in FirebirdSql

SET TERM ^ ;

CREATE PROCEDURE sprocname
 ( id int ) 
RETURNS 
 ( gcode int, errmsg varchar(250) )
AS 
BEGIN
  gcode = 0;
  errmsg = '';
  -- do procedure code here


  WHEN ANY DO
  BEGIN 
    gcode = gdscode; -- ??
    errmsg = ??;
  END
  SUSPEND;
END^

SET TERM ; ^