Pass on download or installation errors from WebstoreInstaller to the CompleteInstall function.

chrome/test/data/extensions/api_test/webstore_private/empty.js

+// 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.
+
+var manifest = getManifest();
+getIconData(function(icon) {
+
+  // Begin installing.
+  chrome.webstorePrivate.beginInstallWithManifest3(
+      {'id': extensionId,'iconData': icon, 'manifest': manifest },
+      function(result) {
+        assertNoLastError();
+        assertEq(result, "");
+
+        // Now complete the installation.
+        var expectedError = "Package is invalid: 'CRX_HEADER_INVALID'.";
+        chrome.webstorePrivate.completeInstall(extensionId,
+                                               callbackFail(expectedError));
+  });
+});