Fix the showing of the VALIDATE message.

Fixes bug #17405 again.

changelog

@@ -18,6 +18,8 @@ Version 1.9.4+svn:
    * Implemented: the helptips.
    * Changed: the scroll wheel, in gui2 code, now also follows the mouse focus
      instead of the keyboard focus.
+   * Fixed again: Not showing the twml_exception dialog when gui2 was called
+     from Lua (bug #17405).
  * WML engine:
    * Allow [color_range] and [color_palette] nodes to be inserted at top-level
      by add-ons to globally define custom ranges and palettes.

src/wml_exception.hpp

@@ -23,6 +23,7 @@
 #define WML_EXCEPTION_HPP_INCLUDED
 
 #include "config.hpp"
+#include "lua_jailbreak_exception.hpp"
 
 #include <string>
 
@@ -84,6 +85,7 @@ void wml_exception(
 
 /** Helper class, don't construct this directly. */
 struct twml_exception
+	: public tlua_jailbreak_exception
 {
 	twml_exception(const std::string& user_msg, const std::string& dev_msg)
 		: user_message(user_msg)
@@ -111,6 +113,8 @@ struct twml_exception
 	 *  @param disp         The display object to show the message on.
 	 */
 	void show(display &disp);
+private:
+	IMPLEMENT_LUA_JAILBREAK_EXCEPTION(twml_exception)
 };
 
 /**