fixed throw statement in Function::operator()

Function.hpp

@@ -57,7 +57,7 @@ public:
     Ret operator()(Args... args) const
     {
         if (_is_null)
-            throw std::bad_function_call("Error: call to nullptr Function!");
+            throw std::bad_function_call();
         return _fun->call(std::forward<Args>(args)...);
     }