That is the exact thing that happened to me as well- same error when I went through "Steam" earlier. I've been D\L theHunter through theHunter website, started at 4:23 PM it is now 9:14 PM- that is 5 hours and 4 hours earlier when I tried the Patching through "Steam". So, 9 hrs and I still can't play the game. And I've probably used up a bunch of Data. With only 30 Gb/s alloted through my service provider, I use up way more than I care to say, with every patch. Very annoying and almost to the point of a major rant in this regard! Not a happy hunter.
Yes, for the first time I'm aware of, the game executable from Steam and the Web install are different. This breaks the game folder copy workaround. Interestingly, the only file that is different is "theHunter.exe" which is only a small 13 MB file. All of the other files in the game folders are identical (all 4 GB of them). That one tiny difference is the problem.
If you, or anyone, wants a good copy of the Web version of "theHunter.exe", send me a PM and I can give you a link to download it from my NAS. Its a small 13MB file, and should take less than a minute to download. You would then do the Steam copy shown in the video first, and then replace the Steam version of "theHunter.exe" with the downloaded one you got from me. I've tried it and it works.
The only problem may be if your anti-virus quarantines the file because it's an executable from and "Unknown Source", namely, ME... If your anti-virus does this, it should give you an option to "trust" the file, but I'm only familiar with Norton Anti-Virus, so I'm not sure what it looks like from other anti-virus packages.
Of course, you'll also need to trust me, but hey....