...Yeah, I've never seen that error either.
Worth a mention, however: Win 10 definitely saves your old install (puts everything in \Windows.old) so that you can revert it if necessary. This is reliable in my experience (when Win 10 has a new build, it upgrades and leaves the old build in \Windows.old ... since I have 10 installed on a rather small partition, if I didn't clean up the temp files before it auto-upgraded, the upgrade would fail, and automatically roll back. This happened at least 7 times, probably more like a dozen. Zero issues rolling back.)
As well, you can have Win 10 take an image backup of your current install. So there's that.