remove JSON parameter list from challenge module and add unified return interfaces for challenge modules

Change-Id: I0e69f7f8ef2ea36f38f11815aa980928b049cdc7
diff --git a/src/ndncert-common.hpp b/src/ndncert-common.hpp
index cbbba39..6727a88 100644
--- a/src/ndncert-common.hpp
+++ b/src/ndncert-common.hpp
@@ -149,6 +149,7 @@
 std::string statusToString(Status status);
 
 enum class Error : uint16_t {
+  NO_ERROR = 0,
   BAD_INTEREST_FORMAT = 1,
   BAD_PARAMETER_FORMAT = 2,
   BAD_SIGNATURE = 3,