I would like to be able to add an optional handler that is called in case a Gateway, Script, On Entry, or On Exit task action code throws some kind of exception. I would like to be able to configure this per task/gateway.

We frequently end up with exceptions that bubble through to break process instances, however hard we try to be consistent and test thoroughly. Because a throwable will crash a process instance, we can't add an error flow to the workflow to handle it, it is too late by then.

So in our actions we can code:
try {
dosomething that raises an exception;
} catch (Throwable t) {
dosomething to handle the error;
}

But I would prefer the catch block to be a parameter of the task, that always catches errors e.g. NPEs. That way the presence of the handling code is clearer when reviewing.