dont fail copy on not found error

sectorbuilder.go

@@ -724,6 +724,10 @@ func (sb *SectorBuilder) ImportFrom(osb *SectorBuilder, symlink bool) error {
 
 	val, err := osb.ds.Get(lastSectorIdKey)
 	if err != nil {
+		if err == datastore.ErrNotFound {
+			log.Warnf("CAUTION: last sector ID not found in previous datastore")
+			return nil
+		}
 		return err
 	}