[Sync] Introduce ModelError for USS error handling.

components/sync/model/model_error.h

+// 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.
+
+#ifndef COMPONENTS_SYNC_MODEL_MODEL_ERROR_H_
+#define COMPONENTS_SYNC_MODEL_MODEL_ERROR_H_
+
+#include <string>
+
+#include "base/location.h"
+
+namespace syncer {
+
+// A minimal error object for use by USS model type code.
+class ModelError {
+ public:
+  // Creates an un-set error object (indicating an operation was successful).
+  ModelError();
+
+  // Creates a set error object with the given location and message.
+  ModelError(const tracked_objects::Location& location,
+             const std::string& message);
+
+  ~ModelError();
+
+  // Whether this object represents an actual error.
+  bool IsSet() const;
+
+  // The location of the error this object represents. Can only be called if the

[skym, 2017/01/09 18:20:28]

Hmm, this is actually looking a lot like an optional.
https://cs.chromium.org/chromium/src/base/optional.h?q=optional&sq=package:ch...

WDYT?

[maxbogue, 2017/01/09 21:29:33]

I like the idea, but I'd like to leave it for a follow-up CL to try to
implement.

+  // error is set.
+  const tracked_objects::Location& location() const;
+
+  // The message explaining the error this object represents. Can only be called
+  // if the error is set.
+  const std::string& message() const;
+
+ private:
+  bool is_set_;
+  tracked_objects::Location location_;
+  std::string message_;
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_MODEL_MODEL_ERROR_H_