I haven't had any similar problems since making a breakthrough, completing the upgrade, and now (five months on) I can't remember what helped. The guy who sold me my laptop had already done a complete install and upgrade, and he said it worked first go
for him.
I think, but can't prove, that the original get-to-50%-and-crash might be related to my internet connection - very high speed cable, but with a serious ISP-induced DSN resolution problem. I eventually made that problem go away. A lot of things work better
and faster now.
Because the network fixes were done after getting to 8.1, the only way to prove that the original problem was network-related would be to go back and reinstall Win8 from scratch and then repeat the upgrade process. Given that my system is stable, that doesn't
have a lot of appeal.
On balance, my guess is that the likely culprit is some combination of a bad network connection (ie: a local user problem), and poor ungraceful error recovery in the download process (ie: a Microsoft coding problem).
What's completely culpable is the error message which is totally accurate, but doesn't tell you anything useful or helpful.
John