The issue is that from the perspective of the hypervisor, the virtual instance is running -- the virtual hardware is "powered on" and has started executing instructions. There is no good way for Nova (or Horizon) to detect that something has gone wrong after the system has started booting.

If you need to detect this sort of situation, you probably want to implement some sort of monitoring solution that can actually monitor the services you expect to be running (so that it can alert you, for example, that your web server is not responding).