Update cloud migrate group backup access denied message

Signed-off-by: Grant Griffiths <[email protected]>

api/server/sdk/cloud_backup.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/libopenstorage/openstorage/api"
 	"github.com/libopenstorage/openstorage/volume"
+	"github.com/portworx/kvdb"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
 )
@@ -133,7 +134,11 @@ func (s *CloudBackupServer) GroupCreate(
 
 		// Check ownership for this intersection
 		if err := checkAccessFromDriverForVolumeIds(ctx, s.driver(ctx), volumesToCheck, api.Ownership_Read); err != nil {
-			return nil, err
+			if err == kvdb.ErrNotFound {
+				return nil, err
+			}
+
+			return nil, status.Errorf(codes.PermissionDenied, "Access denied to at least one volume in group %s", req.GetGroupId())
 		}
 
 	// Only a slice of VolumeIDs provided