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>

src/pkg/gcs/gcs_client.go

diff --git a/src/pkg/gcs/gcs_client.go b/src/pkg/gcs/gcs_client.go
index 57709f3..7c6b9c6 100644
--- a/src/pkg/gcs/gcs_client.go
+++ b/src/pkg/gcs/gcs_client.go
@@ -323,6 +323,13 @@
 		return nil, err
 	}
 
+	if len(descendants) == 0 {
+		return nil, fmt.Errorf(
+			"no descendants found for %s; either the bucket does not exist, or you do not have the required permissions to access it",
+			bucket.URI(bucketDir),
+		)
+	}
+
 	var downloads []ObjectDownload
 	for _, objectName := range descendants {
 		// In a GCS bucket, the paths some/path and /some/path are different.