proxy mirror exits 0 when downloads fail #500

runMirror in cmd/proxy/main.go prints N failed and lists the errors, then returns normally, so the exit status is 0. A CI step that prewarms the cache with proxy mirror passes even when every download fails, for example on a PyPI PURL that the resolver rejects (#414).

runMirror should exit with status 1 when progress.Failed > 0, so callers can detect a partial mirror from the exit status.

Related: #415, #478, #493