Place an indicator on the panel. Set the text to HUGE and RED, and set its value to "STOP, ERROR DETECTED!". Make it not visible. When you get an error, use the "Visible" property node to display it.
To learn more about LabVIEW it is recommended that you go through the tutorial(s) and look over the material in the NI Developer Zone's Learning Center which provides links to other materials and other tutorials. You can also take the online courses for free.
