[Sync] Introduce ModelError for USS error handling.

components/sync/model/model_error.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/sync/model/model_error.h"
+
+namespace syncer {
+
+ModelError::ModelError() : is_set_(false) {}
+
+ModelError::ModelError(const tracked_objects::Location& location,
+                       const std::string& message)
+    : is_set_(true), location_(location), message_(message) {}
+
+ModelError::~ModelError() = default;
+
+bool ModelError::IsSet() const {
+  return is_set_;
+}
+
+const tracked_objects::Location& ModelError::location() const {
+  DCHECK(IsSet());
+  return location_;
+}
+
+const std::string& ModelError::message() const {
+  DCHECK(IsSet());
+  return message_;
+}
+
+}  // namespace syncer