manager: detect lost connection when executor gets killed

[xairy]

When fuzzing the USB HID drivers, I noticed that most of the time is being spent on rebooting VMs without any detected crashes and before the normal 1 hour timeout passes:

2024/07/11 15:34:03 vm-1: running for 48.399456145s, restarting
...

What happens is that due to #5027, the executor process gets killed all the time due to Ctrl+Alt+Del.

Fixing that issue should solve the problem of executor getting killed, however, I think syz-manager needs to detect such cases as lost connection to allow identifying and fixing them if they happen again in the future (or possibly already happening right now).

The reason syz-manager ignores the executor getting killed is because the executor prints SYZ-EXECUTOR: PREEMPTED before exiting:

# ./syz-execprog -enable=usb -repeat=0 -procs=1 -debug ./log
...
SYZ-EXECUTOR: PREEMPTED (errno 9)
executor process exited: exit status 1

Thus, extractError in vm/vm.go just exits on the executorPreemptedStr check.

The comment in executor/executor_runner.h says that this is related to GCE VM preemption:

static void SigintHandler(int sig)
{
	// GCE VM preemption is signalled as SIGINT, notify syz-manager.
	exitf("SYZ-EXECUTOR: PREEMPTED");
}

But this is not the case here, as I'm using QEMU.

Commenting out the executorPreemptedStr check in extractError makes syz-manager to detect the lost connection as expected, but I assume this is not the proper fix.