Thanks Joram
If we throw BPMNError in the JavaScript, there is no data in DeadLetterQueue for the process instance. If we avoid the try and catch, we do have an entry in DeadLetterQueue. But then all the variables are lost. There was a suggestion to add try and catch inside the script and throw BPMN error. We did that, but now we do not have any information on the error. The Sequence has reached the boundary delegate we added to the sub process, but how to get the reason for failure (There is no data in dead letter queue, meaning to say there is no row for the process instance id in act_ru_deadletter_job)