Use the user error action.

@karlenigma The problem with that approach is that it takes you to another screen (error screen), which loses any change that you made when you hit the back button.

Ah yes. Sorry was thinking about client script save record. I would use the save record option then before user save I think it is.

¿Can you extend on the save record option?, ¿where is it on the Workflow?

I'm not in my office so couldn't say but I would checkout the suiteflow user guide.