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/transport/unix-transport.cpp b/src/transport/unix-transport.cpp
index ad6db2c..4dc636a 100644
--- a/src/transport/unix-transport.cpp
+++ b/src/transport/unix-transport.cpp
@@ -30,11 +30,11 @@
     {
       return parsed.get<std::string>("unix_socket");
     }
-  catch (const boost::property_tree::ptree_bad_path& error)
+  catch (boost::property_tree::ptree_bad_path& error)
     {
       // unix_socket not present, continue
     }
-  catch (const boost::property_tree::ptree_bad_data& error)
+  catch (boost::property_tree::ptree_bad_data& error)
     {
       throw ConfigFile::Error(error.what());
     }