Installation of macOS High Sierra Update 10.13.4 failed; how do I recover without losing data?
Fortunately I was able to simply reinstall macOS High Sierra by booting into Recovery Mode (Command ⌘ + R) and choosing Reinstall macOS High Sierra.

I did not erase my drive first since system volume was intact (no errors after running Disk Utility). I simply reinstalled the OS and I was able to log back in with all my data, settings, and apps still there just like before. 

Also, this installed the 10.3.4 version of macOS High Sierra, so I don't have to go through the update process again.