Instructions to reproduce: 1. Build Tor Browser via `tor-browser-build`. 2. Right before rbm tries to download a `.sig` file, shut off the network connection. Expected results: rbm should not write a `.sig` file. Future invocations of rbm once network connection is restored should download the `.sig` file. Observed results: rbm writes a `.sig` file with size 0B. When the network connection is restored and rbm is run again, rbm does not retry downloading the `.sig` file, and instead says something like `Error: File llvm-8.0.0.src.tar.xz is not signed with a valid key`. Manually deleting the 0B `.sig` file allows rbm to function properly again.