Give a better error for download-ci-llvm if .xz is not supported
Give a better error for download-ci-llvm if .xz is not supported

Previously:

```
curl: (22) The requested URL returned error: 404
failed to run: curl -# -y 30 -Y 10 --connect-timeout 30 --retry 3 -Sf -o /tmp/tmp6ptXJV https://ci-artifacts.rust-lang.org/rustc-builds/99ad5a1a2824fea1ecf60068fd3636beae7ea2da/rust-dev-nightly-x86_64-unknown-linux-gnu.tar.gz
```

Now:

```
error: XZ support is required to download LLVM
help: consider disabling `download-ci-llvm` or using a different version of python
Build completed unsuccessfully in 0:00:00
```

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust