Like I already posted, these steps got Windows 10 to start properly installing for me:
What this error means is, your Windows 10 installation folder is unfinished and/or corrupted. Now, how do you fix that?
1. Go into "C:\Windows\SoftwareDistribution\Download" and delete everything in that folder.
2. Now, run the command prompt as an administrator. Type in "wuauclt.exe /updatenow".
3. Go to your Control Panel > Windows Update and your Windows 10 should start re-downloading from scratch, hopefully without flaws this time