[Sync] Introduce ModelError for USS error handling.

components/sync/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/buildflag_header.gni")
 import("//build/config/features.gni")
 import("//components/sync/protocol/protocol_sources.gni")
 import("//testing/test.gni")
 
 static_library("sync") {
@@ skipping 418 matching lines @@
     "model/data_type_error_handler_impl.cc",
     "model/data_type_error_handler_impl.h",
     "model/entity_change.cc",
     "model/entity_change.h",
     "model/entity_data.cc",
     "model/entity_data.h",
     "model/local_change_observer.h",
     "model/metadata_batch.cc",
     "model/metadata_batch.h",
     "model/metadata_change_list.h",
+    "model/model_error.cc",
+    "model/model_error.h",
     "model/model_type_change_processor.cc",
     "model/model_type_change_processor.h",
     "model/model_type_debug_info.cc",
     "model/model_type_debug_info.h",
     "model/model_type_store.cc",
     "model/model_type_store.h",
     "model/model_type_sync_bridge.cc",
     "model/model_type_sync_bridge.h",
     "model/mutable_data_batch.cc",
     "model/mutable_data_batch.h",
@@ skipping 685 matching lines @@
     deps = [
       ":fake_server_jni",
       ":sync",
       ":test_support_fake_server",
       "//base",
       "//testing/gtest",
       "//url:url",
     ]
   }
 }