In previous versions of macOS you could show the installer log during system installation with the keyboard shortcut ⌘L. Since macOS Sierra, much of the system installation process happens on a black screen with a white Apple logo and a progress bar.
On that screen, you can use the keyboard shortcut ⇧⌃⌥⌘W, and it will switch back to traditional UI with the grey window. There you can then use ⌘L to show the log window again.
(Double mouse pointers are because I was running the installer in a VM to record the screen.)