Continued working on scanner.

We're now using exceptions for errors, and scanning/pushing tokens is exception-safe (using a set of "limbo tokens").

exceptions.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <exception>
+
+namespace YAML
+{
+	class Exception: public std::exception {};
+
+	class UnknownToken: public Exception {};
+	class IllegalBlockEntry: public Exception {};
+	class IllegalMapKey: public Exception {};
+	class IllegalMapValue: public Exception {};
+	class IllegalScalar: public Exception {};
+}