Custom Error Message

I am in the process of trying to write code to verify the fields entered for a listing. I have a function that runs at (or hooks into) acdap_listing_form_after_save. All works as planned but I can not get the error message to display. The code returns to the save_listing function and the wp_redirect( $redirect_url ) runs and my error messages never get displayed. Can I get someone to help me with this? I do not know php in depth but when I comment out the redirect, I can get the message but I do not want to change your code in case of updates.