Improve inaccessible plugin marketplace errors and private GitHub authentication

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: Insiders
- OS Version: macOS

Steps to Reproduce:

1. Configure or install a plugin marketplace hosted in a private GitHub repository.
2. Attempt installation from the desktop customizations UI without Git credentials available to the local Git process.

The notification currently exposes the full `git clone` command, cache path, and raw `Repository not found` stderr. This is noisy and does not explain that GitHub returns the same response for a missing repository and an account without access.

Expected UX:
- Explain that the marketplace could not be accessed because it may be private, missing, or the current GitHub account lacks permission.
- Offer an actionable GitHub sign-in/retry flow for GitHub HTTPS sources.
- Keep **Show Git Output** as a secondary diagnostic action rather than putting raw command output in the main notification.
- Preserve distinct, actionable messages for cancellation, authentication/access, missing repository, network, and invalid repository/ref failures.