exceptions: Make Tlv::Error a base class for all packet-processing exceptions

This commit also includes a number of code style fixes.

Change-Id: I44f83915e733b43d5f43b4266902c8262e928d91
Refs: #1528
diff --git a/src/security/validator-null.hpp b/src/security/validator-null.hpp
index f389d4c..1bb6d01 100644
--- a/src/security/validator-null.hpp
+++ b/src/security/validator-null.hpp
@@ -13,7 +13,8 @@
 
 namespace ndn {
 
-class ValidatorNull : public Validator {
+class ValidatorNull : public Validator
+{
 public:
   virtual
   ~ValidatorNull()