Scriptrunner validator show error on transition without a screen

[TJ-LDS]

I have a transition that does not use a screen. The transition has a Scriptrunner validator that runs. I need this validator to pop up with a custom error message if the validation fails, but at the moment, I can only get it to show this generic one:

"It seems that you have tried to perform an illegal workflow operation.

If you think this message is wrong, please contact your Jira administrators."

I have tried using the invalidInputException variable, but ScriptRunner tells me that that variable is undeclared. If I save the script anyway despite this error, I still do not see my custom error string.

I can't find any complete examples of how to use invalidInputException. Are you just supposed to assign it a new InvalidInputException then return False? Or are you supposed to throw it once it's assigned? Or something else?