After I create the file 'unofficial-hdhomerun-dvr-repo.list' in /etc/apt/sources.list.d containing 'deb https://unofficial-hdhomerun-dvr-repo.s3.us-east-1.amazonaws.com/deb stable non-free' and run sudo apt-get update, I see this:
E: The repository 'https://unofficial-hdhomerun-dvr-repo.s3.us-east-1 ...