security: rename NO_SIGNATURE error code to MALFORMED_SIGNATURE

And move the documentation for all validation error codes to doxygen

Change-Id: Ibf4b507e3a544f2978192fbfad20cd7dc486818f
diff --git a/tests/unit/security/validation-error.t.cpp b/tests/unit/security/validation-error.t.cpp
index 6c1b5f7..c4a1a08 100644
--- a/tests/unit/security/validation-error.t.cpp
+++ b/tests/unit/security/validation-error.t.cpp
@@ -22,6 +22,7 @@
 #include "ndn-cxx/security/validation-error.hpp"
 
 #include "tests/boost-test.hpp"
+
 #include <boost/lexical_cast.hpp>
 
 namespace ndn {
@@ -37,17 +38,22 @@
   ValidationError e1{ValidationError::INVALID_SIGNATURE};
   BOOST_CHECK_EQUAL(e1.getCode(), 1);
   BOOST_CHECK_EQUAL(e1.getInfo(), "");
-  BOOST_CHECK_EQUAL(boost::lexical_cast<std::string>(e1), "Invalid signature");
+  BOOST_CHECK_EQUAL(boost::lexical_cast<std::string>(e1), "Signature verification failed");
 
-  ValidationError e2{ValidationError::NO_SIGNATURE, "message"};
+  ValidationError e2{ValidationError::MALFORMED_SIGNATURE, "message"};
   BOOST_CHECK_EQUAL(e2.getCode(), 2);
   BOOST_CHECK_EQUAL(e2.getInfo(), "message");
-  BOOST_CHECK_EQUAL(boost::lexical_cast<std::string>(e2), "Missing signature (message)");
+  BOOST_CHECK_EQUAL(boost::lexical_cast<std::string>(e2), "Missing or malformed signature (message)");
 
   ValidationError e3{65535, "other message"};
   BOOST_CHECK_EQUAL(e3.getCode(), 65535);
   BOOST_CHECK_EQUAL(e3.getInfo(), "other message");
   BOOST_CHECK_EQUAL(boost::lexical_cast<std::string>(e3), "Custom error code 65535 (other message)");
+
+  ValidationError e4{200};
+  BOOST_CHECK_EQUAL(e4.getCode(), 200);
+  BOOST_CHECK_EQUAL(e4.getInfo(), "");
+  BOOST_CHECK_EQUAL(boost::lexical_cast<std::string>(e4), "Unknown error code 200");
 }
 
 BOOST_AUTO_TEST_SUITE_END() // TestValidationError