Update install failing macOS Seirra

I was on Sierra public beta 1, then on the lock screen, while trying to open the Guest account, it restarted automatically. Now it won't boot up, the installer is failing with the following error:

Is there any way I can stop the update from installing itself and opening the installed OS. I don't have a time machine backup. Is there a way to take one from inteet recovery or a bootable disk?

The system is a 13" MBP Retina.

Recent Questions...