The only to throw an exception from a sub process is by leveraging the Exception event (from the event view).
You need to throw the exception, by dragging the Exception event and choose Throw into your subprocess. Then you need to add an exception "catcher" in the parent process to catch the Exception.
Where are that exception catcher? Does it only works for custom exceptions?
any example of a process with this?
If you go on the Event view, you'll see an Exception category. Use the Exception handler from there.