....so, after reading about the possibility of email data loss in the Mail.app after upgrade to Catalina, I've taken a particularly cautious approach to installing macOS 10.15. I pre-stripped my existing Mojave SSD of as many 32 bit tidbits and apps as I could find, ensured that I had not one, but two Time Machine backups PLUS a CCC clone on a fresh backup drive...then found that my every attempt to download the upgrade installer was thwarted by the same sort of failure issues that iBozz was experiencing! In my case it was even a bit worse in that I couldn't even get the download process to begin. I even tried joemikeb's suggested process from the Recovery Drive and had the same issue....on several different machines and accounts. This led me to suspect some form of connectivity issue, so I fired up my iPhone X hot spot and that worked!
Going back to continue picking the scab after a successful download over cellular, I tried a restart into SafeBoot mode to eliminate any third party system level conflicts and had a partial improvement, but still hung and error'd out after only a few hundred MB's.....then I tried the iBozz interrupt and that got me a few hundred MB's further.....then I pointed my VPN to a San Francisco server while still booted in SafeBoot mode and having stutter-stepped up to a few hundred MB's of the 8.09 GB upgrade total, et voila! That finally got me a normally-paced successful download/upgrade installer in my Applications directory.
That ran just fine and I am now on Catalina......and I still don't know why the connectivity issues existed. Router resets, local pings, Speed Test throughput, other downloads, other Software Update system updates, direct Cat6 connection to the router, etc all failed to resolve the Catalina upgrade failure for me until using SafeBoot mode and a VPN to a San Francisco exit point, then it ran normal.
