Exit with status 1 when proxy mirror fails to mirror a package

[cestercian]

Fixes #500

runMirror printed N failed and the error list, then returned normally, so proxy mirror exited 0 even when every download failed. A CI step that prewarms the cache could never notice a partial mirror.

Now, after the summary and the error list are printed (same output as before), the command exits with status 1 if progress.Failed > 0. The decision is a small mirrorExitCode(progress) helper, so it's easy to test, and the DB is closed before os.Exit like the other exit paths in runMirror. A run with no failures, including one where everything was already cached, still exits 0. --dry-run is untouched.

I also added one sentence to the mirror section of the README about the exit status.

Repro before the change (offline, using an ecosystem the resolver rejects):

$ proxy mirror --storage-url file:///tmp/pxt/store --database-path /tmp/pxt/db.sqlite pkg:bogus/foo@1.0.0
Mirror complete: 0 downloaded, 0 skipped (cached), 1 failed, 0 B total

Errors:
  bogus/foo@1.0.0: resolving download URL: unsupported ecosystem: bogus
$ echo $?
0

pkg:pypi/requests@2.0.0 from #414 did the same. With this change both exit 1, and pkg:npm/lodash@4.17.21 still mirrors and exits 0.

Tests, in a new cmd/proxy/mirror_test.go:

• TestMirrorExitCode: table test for the helper (nil, all mirrored, all cached, one failure, all failed)
• TestMirrorExitsNonZeroWhenDownloadsFail: runs proxy mirror pkg:bogus/foo@1.0.0 in a subprocess against a temp sqlite DB and file storage, the same re-exec pattern TestServeHelpListsUpstreamEnvironmentVariables uses. It checks for exit status 1 and that the failure is still reported. No network needed.

go test ./...
go vet ./cmd/...
golangci-lint run ./cmd/proxy/...   # 0 issues