changing retval in case of failed cd cmd
if filename does not exist, siril_change_dir(filename, NULL) returns 2, ie CMD_NO_WAIT which is "no error"; in a script such an error should be considered fatal and end the script. So we return CMD_DIR_NOT_FOUND instead.
This might prevent multiple error messages to pop up during script execution, while the faulty command might just be the first "cd" in the script.