Add back `help_on_error` for download-ci-llvm

[jyn514]

I accidentally missed this when porting it to rustbuild: https://github.com/rust-lang/rust/pull/95170/files#diff-9e25a089f077eae8c8cefe9586cc07498e64cd950be4c954cb90e4245e5f9fc3L634-L651

It would be nice to add it back.

Mentoring instructions: Add a new help_on_error parameter to Builder::download_component_helper and pass it down from download_ci_llvm:

rust/src/bootstrap/builder.rs

Line 872 in 00bb4df

pub(crate) fn download_component(&self, base: &str, url: &str, dest_path: &Path) {

rust/src/bootstrap/native.rs

Line 182 in 00bb4df

builder.download_component(base, &format!("{}/{}", url, filename), &tarball);

This code moved around in #96687 so you may want to base your changes off that PR until it lands.

@rustbot label +E-easy +E-mentor +A-rustbuild

[jyn514]

Please also remove the dead code from bootstrap.py at the same time:

rust/src/bootstrap/bootstrap.py

Lines 200 to 201 in 00bb4df

	if help_on_error is not None:
	err += "\n" + help_on_error

[spiral-ladder]

@rustbot claim