Fix bugs, resource leaks, and test issues in cmd/desync

[folbricht]

Summary

• store.go: Fix wrong variable in S3 bucket lookup error message — indexStoreFromLocation printed the nil IndexStore variable s instead of the lookup string ls (the equivalent code in storeFromLocation was correct)
• chunkserver.go, mount-index.go: Fix errors.Wrapf passing err as the format argument instead of opt.storeFile — the error was printed twice while the filename was lost
• info.go: Fix missing closing quote in unsupported format error string ('%s → %q)
• cat.go, inspectchunks.go: Close output file when writing to disk — os.Create was called but the file was never closed
• chunkserver.go: Initialize loggingResponseWriter.statusCode to http.StatusOK so handlers that write a body without explicitly calling WriteHeader() log 200 instead of 0
• config.go: Write config JSON to stdout instead of stderr in read mode
• prune.go: Check error return from fmt.Fscanln to prevent infinite loop when stdin is closed
• chunkserver_test.go, indexserver_test.go: Replace require.NoError (calls t.FailNow) with t.Errorf in server goroutines, per project convention (PR Do not call t.Fatal() from non-main goroutines #291)
• verifyindex_test.go: Remove no-op require.Contains(t, b.String(), "") assertions that always pass

Test plan

• go build ./cmd/desync succeeds
• go test ./... passes