So basically what you want is the call an executable file, wait while it runs and then trap any errors it produced and return them back to the asp page so you can do what you like with it,.... right?
With a simple exe you can't really do this...
What you could do is get your exe to generate a results file or something. Before you execute your exe you delete any results files that exist, then run your exe and loop waiting for the results file to turn up.