Simulator throws an exception when forgetting state with the empty set

[apasel422]

Due to

attribution/impl/src/backend.ts

Lines 625 to 627 in 74fa8b6

	if (sites.size === 0) {
	throw new RangeError("need to specify at least one site when forgetting");
	}

The exception itself makes sense in terms of business logic, but the issue isn't propagated to the UI, so the user has no indication of why their intended operation had no effect.