Tor Browser 4.5 displays signature validation error during update

I suspect this is due to the fact that we allow an update to proceed if it is signed with either my mar signing key or gk's mar signing key, but nonetheless TBB 4.5 displays two error messages while updating on Linux: "ERROR: Error verifying signature" "ERROR: Not all signatures were verified".

We should ensure the signature validation behavior is actually correct, and if so remove these error messages for the stable release.