You need to add -diag in the wdl/starter file iirc (search for wdl in the manual for more info and an example).
You can also try setting diag_mode in your script but I think that is more for during the game and not on startup.

Example from my own game / wdl file;

WINDOW WINSTART
{
MODE STANDARD;
COMMAND "-diag";
}

Also if you use the default starting up screen it also shows the errors (but that ofcourse means not having a fancy custom startup screen).