Fix failure status bug: when status is failure, request should be stopped

Change-Id: I699d9ae6bdc34ac67d9a8b3c69e9d789f9709c6d
diff --git a/src/challenge-module.hpp b/src/challenge-module.hpp
index 7e36d4c..af0d40b 100644
--- a/src/challenge-module.hpp
+++ b/src/challenge-module.hpp
@@ -173,6 +173,7 @@
   static const std::string WAIT_SELECTION;
   static const std::string SUCCESS;
   static const std::string PENDING;
+  static const std::string FAILURE;
 
 private:
   typedef function<unique_ptr<ChallengeModule> ()> ChallengeCreateFunc;