Incomplete cell hangs kernel

[vors]

Incomplete cells currently hang the kernel. I.e. (
Unfortunately, powershell input redirection doesn't allow to enter \n few more times to get incomplete input error. But we should be able to handle it better.

[vors]

I put a mitigation detection in the kernel, because we can detect the hang, we just cannot do anything about it: there is nothing we can send powershell process to abort operation or do anything else to get the error message.