Joshua Nelson b5019d6ec3 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
```
2021-01-01 10:41:56 -05:00
..
2020-12-29 13:37:21 +09:00
2020-12-30 19:14:15 +00:00

This directory contains the source code of the rust project, including:

  • The test suite
  • The bootstrapping build system
  • Various submodules for tools, like rustdoc, rls, etc.

For more information on how various parts of the compiler work, see the rustc dev guide.