feat: Added warning when failing to update index cache

[ranger-ross]

What does this PR try to resolve?

Fixes #13712

Adds a warning if there is an error updating the index cache.
It also attempts to avoid warning spam as requested in this comment

Below is an example output

    Updating crates.io index
warning: failed to write cache, path: /home/ross/.cargo/registry/index/index.crates.io-1949cf8c6b5b557f/.cache/ba/se/base64, error: Permission denied (os error 13)
   Compiling serde v1.0.217
   Compiling r-efi v5.2.0
   Compiling base64 v0.22.1
   Compiling cargo-13712 v0.1.0 (/home/ross/projects/cargo-13712)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.20s

How should we test and review this PR?

I tested this on my machine by manually changing the owner/permission of the index files

sudo chown root ~/.cargo/registry/index/.../.cache/r-
sudo chmod 700 ~/.cargo/registry/index/.../.cache/r-

# in a project that uses the `r-efi` crate
cargo build 

I wasn't quiet sure about the best way to add a test to the testsuite for this. 😅
If there is good way to create a test for this lmk

[rustbot]

r? @weihanglo

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[weihanglo]

AtomicBool should be good enough here. But maybe bool is sufficient and we don't even need to coordinate threads here because most parts of Cargo are single-threaded.

[weihanglo]

We have a similar test, maybe we can take reuse that test to assert their stderr?

cargo/tests/testsuite/registry.rs

Lines 3053 to 3098 in 2939e96

	fn readonly_registry_still_works() {
	Package::new("foo", "0.1.0").publish();
	let p = project()
	.file(
	"Cargo.toml",
	r#"
	[package]
	name = "a"
	version = "0.5.0"
	edition = "2015"
	authors = []
	[dependencies]
	foo = '0.1.0'
	"#,
	)
	.file("src/lib.rs", "")
	.build();
	p.cargo("generate-lockfile").run();
	p.cargo("fetch --locked").run();
	chmod_readonly(&paths::home(), true);
	p.cargo("check").run();
	// make sure we un-readonly the files afterwards so "cargo clean" can remove them (#6934)
	chmod_readonly(&paths::home(), false);
	fn chmod_readonly(path: &Path, readonly: bool) {
	for entry in t!(path.read_dir()) {
	let entry = t!(entry);
	let path = entry.path();
	if t!(entry.file_type()).is_dir() {
	chmod_readonly(&path, readonly);
	} else {
	set_readonly(&path, readonly);
	}
	}
	set_readonly(path, readonly);
	}
	fn set_readonly(path: &Path, readonly: bool) {
	let mut perms = t!(path.metadata()).permissions();
	perms.set_readonly(readonly);
	t!(fs::set_permissions(path, perms));
	}
	}