htlcswitch: use distinct error for unreadable failures

htlcswitch/mock.go

@@ -965,3 +965,18 @@ func (m *mockCircuitMap) NumPending() int {
 func (m *mockCircuitMap) NumOpen() int {
 	return 0
 }
+
+type mockOnionErrorDecryptor struct {
+	sourceIdx int
+	message   []byte
+	err       error
+}
+
+func (m *mockOnionErrorDecryptor) DecryptError(encryptedData []byte) (
+	*sphinx.DecryptedError, error) {
+
+	return &sphinx.DecryptedError{
+		SenderIdx: m.sourceIdx,
+		Message:   m.message,
+	}, m.err
+}