Parse messages.json in ExtensionUnpacker (like we do for manifest)....

chrome/common/extensions/extension_constants.cc

 // Copyright (c) 2009 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 "chrome/common/extensions/extension_constants.h"
 
 namespace extension_manifest_keys {
 
 const wchar_t* kBackground = L"background_page";
 const wchar_t* kBrowserAction = L"browser_action";
@@ skipping 166 matching lines @@
 const char* kInvalidUpdateURL =
     "Invalid value for update url: '[*]'.";
 const char* kInvalidDefaultLocale =
     "Invalid value for default locale - locale name must be a string.";
 const char* kOneUISurfaceOnly =
     "An extension cannot have both a page action and a browser action.";
 const char* kThemesCannotContainExtensions =
     "A theme cannot contain extensions code.";
 const char* kLocalesNoDefaultLocaleSpecified =
     "Localization used, but default_locale wasn't specified in the manifest.";
+const char* kLocalesNoDefaultMessages =
+    "Default locale is defined but default data couldn't be loaded.";
 const char* kLocalesNoValidLocaleNamesListed =
     "No valid locale name could be found in _locales directory.";
+const char* kLocalesTreeMissing =
+    "Default locale was specified, but _locales subtree is missing.";
+const char* kLocalesMessagesFileMissing =
+    "Messages file is missing for locale.";
 const char* kInvalidOptionsPage =
     "Invalid value for 'options_page'.";
 }  // namespace extension_manifest_errors