Rollup merge of #99139 - jyn514:dist-tool-help, r=Mark-Simulacrum

Give a better error when `x dist` fails for an optional tool

Before:
```
thread 'main' panicked at 'Unable to build RLS', dist.rs:42:9
```

After:
```
thread 'main' panicked at 'Unable to build submodule tool RLS (use `missing-tools = true` to ignore this failure)
note: not all tools are available on all nightlies
help: see https://forge.rust-lang.org/infra/toolstate.html for more information', dist.rs:43:9
```

Closes https://github.com/rust-lang/rust/issues/85683 by explaining better why the error is expected.