iMac can't boot (probably failed upgrade)

Hello,

I have been out of home for a few days. Today I have turned on my iMac (first Retina 5K, currently macOS High Sierra), It simply can't boot. After attempt to boot, it displays screen with this text(translation from slovak, sorry for my english):

macOS could not be installed on your computer.

Path /System/Installation/Packages/OSInstall.mpkg is missing or damaged. Quit installer to restart and try again.

So it looks like a failed attempt to upgrade macOS. Any suggestions what should I do now? Hopefully I have one more Mac running, and an USB key which I can reformat if required. However I do not have thunderbolt-thunderbolt cable to connect to my iMac using target disk mode.

I'd prefer to repair existing installation, but if there is no chance, how should I backup up and reinstall iMac?

Thanks,

Stanislav