More useful error messages.

Applied the patch given in jbeder/yaml-cpp#200 with the correct code
style.

src/node_data.cpp

@@ -17,6 +17,7 @@ std::string node_data::empty_scalar;
 
 node_data::node_data()
     : m_isDefined(false),
+      m_mark(Mark::null_mark()),
       m_type(NodeType::Null),
       m_style(EmitterStyle::Default),
       m_seqSize(0) {}
@@ -27,6 +28,10 @@ void node_data::mark_defined() {
   m_isDefined = true;
 }
 
+void node_data::set_mark(const Mark& mark) {
+  m_mark = mark;
+}
+
 void node_data::set_type(NodeType::value type) {
   if (type == NodeType::Undefined) {
     m_type = type;