Improve error message for incomplete toolchains

[Inconnu08]

Improve the error message shown when rustup encounters a toolchain that is missing its manifest.

Interrupted installations can leave a partially installed toolchain behind. Rustup already improves the recovery experience when reinstalling such toolchains (#4725), but users can still encounter a confusing "Missing manifest" error when interacting with the incomplete toolchain before retrying installation.

• Added a test that removes the manifest file from an installed toolchain and verifies the improved error message

Related to #4724
Builds on the interrupted-install UX improvements from #4725

[rami3l]

Thanks for working on this!

View changes since this review

[rami3l]

Suggest adding an extra step to see if the suggestion really works.

[Inconnu08]

Edit: I tried verifying the suggested reinstall step.

In the test environment, rustup toolchain install nightly reports the toolchain as unchanged and does not repair the missing-manifest state.

However, when testing manually with a real dist server, I noticed that rustup component list --toolchain nightly actually triggers a re-sync and repairs the toolchain automatically.

So recovery currently depends on the command used, and toolchain install does not explicitly detect and repair incomplete states.

Given this, I think the current suggestion may be misleading, so I’m considering adjusting the help text to be less prescriptive and soften it.