bpm api to start exception or boundary event?

When the approval step is Looping, Parallel for Each.

Anyone rejects, it will stop the loop, and go to final step.

As I'm using UI5 as UI, how to use api to start exception or boundary event?

I find task api: 

void fail(URI taskInstanceId, Fault fault) throws BPMException

But I'm not sure how it works.

How to customized exception, or how to make boundary event work?

Thanks very much.