AU: detect failure when using public key verification

This makes public key verification non-fatal. A future CL will use
this knowledge to report statistics.

Change-Id: I9440155dd71621662e5c0f4011b3001bbc65e6d7

BUG=chromium-os:13341
TEST=unittests

Review URL: http://codereview.chromium.org/6778029
diff --git a/download_action.cc b/download_action.cc
index d6ae97d..9577047 100644
--- a/download_action.cc
+++ b/download_action.cc
@@ -163,7 +163,8 @@
     if (!install_plan_.is_full_update) {
       if (!delta_performer_->VerifyPayload("",
                                            install_plan_.download_hash,
-                                           install_plan_.size)) {
+                                           install_plan_.size,
+                                           NULL)) {
         LOG(ERROR) << "Download of " << install_plan_.download_url
                    << " failed due to payload verification error.";
         code = kActionCodeDownloadPayloadVerificationError;