Yes, pretty much this is what I meant. So you need to have in the last process (validation task) an end event that is throwing an error. Then the process should have catching event to handle the error, if your requirement is to propagate it further then it should thrown another error event, and so on...
Please note that I refer to BPMN2 processes.
Btw, have you tried to make the end event of the most nested process to be an error event instead of regular?! Or maybe it is already like that but the process image does not illustrate this...
Cheers