fix: surface failed package delete during unpublish

[buley]

Summary

• Treat E404 as success only when the initial packument fetch reports the package is already gone.
• Surface 404 failures from the full-package delete path instead of returning success.
• Add regressions at both the libnpmpublish and CLI layers for a failed full-package delete.

Verification

• source ~/.nvm/nvm.sh && nvm use 22.22.0 >/dev/null && cd /Users/buley/Documents/Code/cli-fix-unpublish-clean && npx tap --no-coverage workspaces/libnpmpublish/test/unpublish.js
• source ~/.nvm/nvm.sh && nvm use 22.22.0 >/dev/null && cd /Users/buley/Documents/Code/cli-fix-unpublish-clean && npx tap --no-coverage test/lib/commands/unpublish.js

Repro

Before this change, npm unpublish <pkg> --force could receive a DELETE .../-rev/... 404 from the registry, still exit 0, and print success because libnpmpublish swallowed any E404 raised anywhere in the unpublish flow.