Make global notify of onremove on a system an error

[Kelwan]

Having an onremove notify that applies to the whole system doesn't really make sense (as it will have no components to filter with). A system using onremove needs to have at least 1 component without, or an error will throw.

BAD:

system RemoveNotify {
	readwrite NotifyComponentA;
	readonly NotifyComponentB;
	readonly NotifyComponentC;
	notify onremove;
}

GOOD:

system RemoveNotify {
	readwrite NotifyComponentA;
	readonly NotifyComponentC;
	notify {
		onremove NotifyComponentB;
	}
}