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/validator.t.cpp b/tests/unit/security/validator.t.cpp
index 842f2f5..d1b746b 100644
--- a/tests/unit/security/validator.t.cpp
+++ b/tests/unit/security/validator.t.cpp
@@ -64,7 +64,7 @@
   BOOST_REQUIRE_NO_THROW(interest.getSignatureValue());
 
   VALIDATE_FAILURE(interest, "InterestSignatureInfo decoding should fail");
-  BOOST_TEST(lastError.getCode() == ValidationError::NO_SIGNATURE);
+  BOOST_TEST(lastError.getCode() == ValidationError::MALFORMED_SIGNATURE);
   BOOST_TEST(face.sentInterests.size() == 0);
 }
 
@@ -259,7 +259,7 @@
   Data data("/Security/ValidatorFixture/Sub1/Sub2/Data");
 
   VALIDATE_FAILURE(interest, "Unsigned");
-  BOOST_TEST(lastError.getCode() == ValidationError::NO_SIGNATURE);
+  BOOST_TEST(lastError.getCode() == ValidationError::MALFORMED_SIGNATURE);
   VALIDATE_SUCCESS(data, "Policy bypasses validation for all data");
   BOOST_TEST(face.sentInterests.size() == 0);
   face.sentInterests.clear();