transaction: Do not make Transaction to implement error interface anymore

As per previous commit, Transaction can't be used anymore as an error
value, but we instead we always return the status code.

transaction_test.go

@@ -512,7 +512,7 @@ func Test_Error(t *testing.T) {
 				if err != nil {
 					var status Error
 					if !errors.As(err, &status) || err.Error() != status.Error() {
-						t.Fatalf("error #unexpected status %v vs %v", err.Error(),
+						t.Fatalf("error #unexpected status %#v vs %#v", err.Error(),
 							status.Error())
 					}
 				}