For those who might be wondering how to create the multi-level progress bars,

This is actually very straightforward.

In the processor script include: 

var child_tracker = this.tracker.createChild('Subprocess with its own bar');

the child_tracker object in this case has the same methods available as the parent. 

Calling child_tracker.fail('some failure message') will bubble up the message to the parent tracker and fail the whole thing. 

 

In the UI Action (or client side script that renders the modal)

    dd.setPreference('sysparm_renderer_expanded_levels', 1);

 This will cause the progress bar to render with the first level expanded initially.