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/signature.hpp b/src/signature.hpp
index 8a20baa..3d3c4c9 100644
--- a/src/signature.hpp
+++ b/src/signature.hpp
@@ -12,14 +12,15 @@
 /**
  * A Signature is storage for the signature-related information (info and value) in a Data packet.
  */
-class Signature {
+class Signature
+{
 public:
-  class Error : public std::runtime_error
+  class Error : public Tlv::Error
   {
   public:
     explicit
     Error(const std::string& what)
-      : std::runtime_error(what)
+      : Tlv::Error(what)
     {
     }
   };