Confusing rustup message: can't find checksums for stable channel

```
-> % multirust update stable
multirust: installing toolchain 'stable'
rustup: gpg available. signatures will be verified
rustup: downloading manifest for 'stable'
rustup: command failed: curl -s -f -O https://static.rust-lang.org/dist/channel-rust-stable.toml.sha256
rustup: couldn't download checksum file 'https://static.rust-lang.org/dist/channel-rust-stable.toml.sha256'
```

There's neither toml file nor checksums for it in `/dist/` directory for stable channel, so it's just confusing during `multirust update stable` invocation.