Improve runtime error handling

include/execute/error.hpp

@@ -1,27 +1,44 @@
 #pragma once
+#include <string>
 
 namespace Execute
 {
-	struct RuntimeError
+	class RuntimeError
 	{
+	protected:
+		std::string message;
 
+	public:
+		std::string const & GetMessage() const;
+
+		RuntimeError();
+		RuntimeError(std::string const & what);
 	};
 
-	struct StackUnderflow : RuntimeError
+	class StackUnderflow : public RuntimeError
 	{
-
+	public:
+		StackUnderflow();
 	};
 
-	struct StackOverflow : RuntimeError
+	class StackOverflow : public RuntimeError
 	{
+	public:
+		StackOverflow();
+	};
 
+	class MissingLabel : public RuntimeError
+	{
+	public:
+		MissingLabel(std::string const & label);
 	};
 
 	namespace Internal
 	{
-		struct BadValueType : RuntimeError
+		class BadValueType : public RuntimeError
 		{
-
+		public:
+			BadValueType();
 		};
 	}
 }