For those who (like me) didn't know:
The GUI installer can be re-started from a terminal. When the installer first loads, press the "back" button. You'll be faced with a blank screen. Right-click on the background or mouse to the lower left corner to see ways to launch a terminal. Once it's up, issue "vinstall". The terminal will disappear and be replaced by the initial screen, but mouse to the lower left to see the minimized terminal, ready to be pulled up again. You can also start other terminals during the process.
Now you can see high-level error messages in the terminal, (u)mount partitions, copy over useful info like terminal pastes, install.log, and subprocess.log.
If mounting partitions temporarily, be sure to umount them as soon as possible. But this is usually done after failed install anyway, so . . .