I have been getting the "Download Failed" error message for the last couple of releases, and I noticed in the error log the installer seemed to be failing to load files into the AppData/Local/Temp folder. So I went in and deleted everything in the Temp folder and then ran the installer again, and it worked this time.
Along the way I had to open Task Manager and close a couple of processes Rhino WIP had left running, otherwise I could not delete the linked files in the Temp folder. (Might a reboot have helped here?)
Just something others might try if they are getting this error message.