Need a way to cancel installer from installscript

At the moment, it doesn't matter what kind of QMessageBox we show from the installscript, it always allows a continuation of the installation after pressing ok.
There are cases when we need to show a messagebox from the script with the installer aborting after pressing ok, for example when some prequisite is missing that needs to be installed before the installer is started.

So, at least a "critical" message should abort the installer, or provide a different way to accomplish that. (But why do we have different severity levels for the messageboxes then?)