[Sync] Refactor sync datatype error handling.

chrome/browser/sync/api/syncable_service.h

 // Copyright (c) 2011 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 CHROME_BROWSER_SYNC_API_SYNCABLE_SERVICE_H_
 #define CHROME_BROWSER_SYNC_API_SYNCABLE_SERVICE_H_
 #pragma once
 
 #include <vector>
 
 #include "base/compiler_specific.h"
 #include "chrome/browser/sync/syncable/model_type.h"
 #include "chrome/browser/sync/api/sync_change_processor.h"
 #include "chrome/browser/sync/api/sync_data.h"
+#include "chrome/browser/sync/api/sync_error.h"
 
 class SyncData;
 
 typedef std::vector<SyncData> SyncDataList;
 
 class SyncableService : public SyncChangeProcessor {
  public:
   // Informs the service to begin syncing the specified synced datatype |type|.
   // The service should then merge |initial_sync_data| into it's local data,
   // calling |sync_processor|'s ProcessSyncChanges as necessary to reconcile the
   // two. After this, the SyncableService's local data should match the server
   // data, and the service should be ready to receive and process any further
   // SyncChange's as they occur.
-  virtual bool MergeDataAndStartSyncing(
+  // Returns: A default SyncError (IsSet() == false) if no errors were
+  //          encountered, and a filled SyncError (IsSet() == true)
+  //          otherwise.
+  virtual SyncError MergeDataAndStartSyncing(
       syncable::ModelType type,
       const SyncDataList& initial_sync_data,
       SyncChangeProcessor* sync_processor) = 0;
 
   // Stop syncing the specified type and reset state.
   virtual void StopSyncing(syncable::ModelType type) = 0;
 
   // Fills a list of SyncData from the local data. This should create an up
   // to date representation of the SyncableService's view of that datatype, and
   // should match/be a subset of the server's view of that datatype.
   virtual SyncDataList GetAllSyncData(syncable::ModelType type) const = 0;
 
   // SyncChangeProcessor interface.
   // Process a list of new SyncChanges and update the local data as necessary.
-  virtual void ProcessSyncChanges(
+  // Returns: A default SyncError (IsSet() == false) if no errors were
+  //          encountered, and a filled SyncError (IsSet() == true)
+  //          otherwise.
+  virtual SyncError ProcessSyncChanges(
       const tracked_objects::Location& from_here,
       const SyncChangeList& change_list) OVERRIDE = 0;
 
  protected:
   virtual ~SyncableService();
 };
 
 #endif  // CHROME_BROWSER_SYNC_API_SYNCABLE_SERVICE_H_