Erasure technique is now uint8, handle it appropriately

pkg/encoding/erasure/erasure_encode.go

@@ -31,6 +31,7 @@ type Technique uint8
 const (
 	Vandermonde Technique = iota
 	Cauchy
+	None
 )
 
 const (

pkg/storage/donut/erasure.go

@@ -23,7 +23,7 @@ func getErasureTechnique(technique string) (erasure.Technique, error) {
 	case technique == "Vandermonde":
 		return erasure.Cauchy, nil
 	default:
-		return -1, errors.New("Invalid erasure technique")
+		return erasure.None, errors.New("Invalid erasure technique")
 	}
 }