[gcs-util] Output stderr when running blobfs-compression This is useful for debugging when blobfs-compression says the delivery blob is invalid. Change-Id: I8f30cb8c13cc0a3cbd82c23142760f1ee94df725 Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/infra/+/860806 Fuchsia-Auto-Submit: Anthony Fandrianto <atyfto@google.com> Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com> Reviewed-by: Sen Jiang <senj@google.com>