create procedure command
(
  in stmt varchar( 4000 )
, out rc int
, out msg varchar( 128 )
)
language sql
begin
 declare sqlcode int default  0 ;
 declare exit handler for sqlexception
  begin
   GET DIAGNOSTICS EXCEPTION  1  msg = MESSAGE_TEXT;
   SET rc=SQLCODE;
  end;
 EXECUTE IMMEDIATE stmt;
 SET rc= 0 ;
end/

statement  1 /

begin atomic
  declare rc int;
  declare msg varchar( 128 );

  if (not exists (select  1  from syscat.triggers where trigname='...' and tabschema='...' and tabname='...')) then
    call command('create trigger ...', rc, msg);
    if (rc!= 0 ) then
      set msg =  'create trigger failed with rc: '||rtrim(char(rc));
      SIGNAL SQLSTATE '75001' SET MESSAGE_TEXT = msg;
    end if;
  end if;
end/

statement  2 /
...