Clear any previous errors if we successfully show a message box

src/video/SDL_video.c

@@ -4945,6 +4945,8 @@ int SDL_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *buttonid)
         if (!*error) {
             SDL_SetError("No message system available");
         }
+    } else {
+        SDL_ClearError();
     }
 
     (void)SDL_AtomicDecRef(&SDL_messagebox_count);