gcs-client: Return error when download dir missing

If DownloadDir is called for a GCS path which has no descendants, this
should be an error since it implies that the corresponding virtual GCS
directory does not actually exist.

BUG=None
TEST=presubmit

Change-Id: I84773248773f93cf7f51166774b5cae8a1db078d
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/163225
Cloud-Build: 228075978874@cloudbuild.gserviceaccount.com <228075978874@cloudbuild.gserviceaccount.com>
Reviewed-by: Angel Adetula <angeladetula@google.com>
Tested-by: Kevin Berry <kpberry@google.com>