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

chrome/common/extensions/extension_constants.cc

Index: chrome/common/extensions/extension_constants.cc
===================================================================
--- chrome/common/extensions/extension_constants.cc	(revision 32098)
+++ chrome/common/extensions/extension_constants.cc	(working copy)
@@ -184,8 +184,14 @@
     "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