For actions, switch bool success into an exit code.

This way we can signal specific error conditions and then
send appropriate events to Omaha from the UpdateAttempter.

BUG=560
TEST=unit tests, gmerged and looked at logs

Review URL: http://codereview.chromium.org/3022008
diff --git a/omaha_response_handler_action.cc b/omaha_response_handler_action.cc
index d93919a..75776ab 100644
--- a/omaha_response_handler_action.cc
+++ b/omaha_response_handler_action.cc
@@ -43,7 +43,7 @@
   LOG(INFO) << "Using this install plan:";
   install_plan_.Dump();
 
-  completer.set_success(true);
+  completer.set_code(kActionCodeSuccess);
 }
 
 bool OmahaResponseHandlerAction::GetInstallDev(const std::string& boot_dev,