better (thread) error handling

ml.h

@@ -67,8 +67,10 @@ private:
 
 
 // An exception thrown by the lisp
-class MlError {
+class MlError : public std::exception {
 public:
+    MlError() = delete;
+
     // Create an error with the value that caused the error,
     // the scope where the error was found, and the message.
     MlError(const MlValue &v, MlEnvironment const &env, const char *msg);
@@ -79,7 +81,10 @@ public:
     ~MlError();
 
     // Get the printable error description.
-    std::string description();
+    std::string description() const;
+
+    // inherited from std::exception 
+    virtual const char* what() const noexcept;
 
 private:
     MlValue *cause;