Update the mainfest_version error message to be more instructive.

chrome/common/extensions/extension_constants.cc

 // 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.
 
 #include "chrome/common/extensions/extension_constants.h"
 
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/string_util.h"
@@ skipping 258 matching lines @@
     "Invalid value for 'app.launch.local_path'.";
 const char kInvalidLaunchWebURL[] =
     "Invalid value for 'app.launch.web_url'.";
 const char kInvalidLaunchWidth[] =
     "Invalid value for 'app.launch.width'.";
 const char kInvalidLaunchWidthContainer[] =
     "Invalid container type for 'app.launch.width'.";
 const char kInvalidManifest[] =
     "Manifest file is invalid.";
 const char kInvalidManifestVersion[] =
-    "Invalid value for 'manifest_version'.";
+    "The 'manifest_version' key must be present and set to 2 (without quotes).";
 const char kInvalidMatch[] =
     "Invalid value for 'content_scripts[*].matches[*]': *";
 const char kInvalidMatchCount[] =
     "Invalid value for 'content_scripts[*].matches'. There must be at least "
     "one match specified.";
 const char kInvalidMatches[] =
     "Required value 'content_scripts[*].matches' is missing or invalid.";
 const char kInvalidMinimumChromeVersion[] =
     "Invalid value for 'minimum_chrome_version'.";
 const char kInvalidName[] =
@@ skipping 197 matching lines @@
 const char kCloudPrintAppId[] = "mfehgcgbbipciphmccgaenjidiccnmng";
 const char kAppsPromoHistogram[] = "Extensions.AppsPromo";
 const char kAppLaunchHistogram[] = "Extensions.AppLaunch";
 #if defined(OS_CHROMEOS)
 const char kAccessExtensionPath[] =
     "/usr/share/chromeos-assets/accessibility/extensions";
 const char kChromeVoxDirectoryName[] = "access_chromevox";
 #endif
 
 }