Animator won't crash anymore when there's an error, and instead displays an error message.

animate2.hpp

@@ -113,6 +113,7 @@ namespace animate2 {
       void play();
       void play_sound();
       void stop();
+      bool has_form(const std::string& as_form);
       void set_form(const std::string& form);
       void apply(sf::Sprite& sprite);
       void update_frame();