Make <AppSystem>.load_from_desktop_file() raise an exception again

Pass the error variable to g_key_file_load_from_data_dirs in
Shell.AppSystem.get_default().load_from_desktop_file again, and
use a try/catch in places.js.

src/shell-app-system.c

@@ -652,7 +652,7 @@ shell_app_system_load_from_desktop_file (ShellAppSystem   *system,
     {
       char *app_path = g_build_filename ("applications", filename, NULL);
       success = g_key_file_load_from_data_dirs (keyfile, app_path, &full_path,
-                                                G_KEY_FILE_NONE, NULL);
+                                                G_KEY_FILE_NONE, error);
       g_free (app_path);
     }