rbm downloads 0B sig file if network drops; rejects sig on next run

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.