Fail nicely when a wrong toolchain is requested

```
phantom ~ $ multirust update default
multirust: installing toolchain 'default'
multirust: gpg available. disabling https (avoids rust#21293)
multirust: interpreting toolchain spec as explicit version
multirust: downloading rust installer from 'http://static-rust-lang-org.s3-website-us-west-1.amazonaws.com/dist/rust-default-x86_64-unknown-linux-gnu.tar.gz'
multirust: downloading 'http://static-rust-lang-org.s3-website-us-west-1.amazonaws.com/dist/rust-default-x86_64-unknown-linux-gnu.tar.gz.sha256' to '/home/flaper87/.multirust/tmp/tmp-8059-1'
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0curl: (22) The requested URL returned error: 404 Not Found
multirust: couldn't download checksum file 'http://static-rust-lang-org.s3-website-us-west-1.amazonaws.com/dist/rust-default-x86_64-unknown-linux-gnu.tar.gz.sha256'
```

I mistakenly did the above and multirust currently just fails to download the `sha` and exits. We should probably check nicely if the toolchain exists (probably execute a `HEAD` call to the `sha` url before downloading) and report an error if it doesn't.