Possible to show a Warning (not an Error) on When and Why?

Summary:

I would like to show a Warning in case the Action Date is not a date in the future.

I tried with Rule Type = "Field Modification Rule Type", Business Object = "When and Why", Field = "Action Date".

While debugging, the code is running but the Page itself does not load (blank page) when triggering the "Add a Pending Worker" action . No error message. Can we do a Warning at this stage? Please note the code it just a test that should always trigger the message.

Content (required):

See Code Snippet.

Version (include the version you are using, if applicable):

22C (11.13.22.07.0)