How to return an error from a make_rtw_hook.m file in Simulink?

I create a custom target_make_rtw_hook.m file for my Simulink external target. How can I end the build when an error occurs, like when a needed file is not present?

command = ([ 'ls needed_file'   ]);
  [status, result] = system(command)
  if status ~= 0
      disp 'Error, file not found!'
      How to stop this make??
  end

Is there a way to do this? And also, how do I get the popup of the diagnostic window like when an error happens?

Thanks in advance, Sietse