TJ,

I don’t think there is any way to change the Apex error message without adding a trigger or custom Apex class.

You may want to use Skuid to adjust what the user can do on the page when the WhoId has already been added to the Campaign.  You could disable or hide the field that allows them to ‘add’ the WhoId to the Campaign.  This will prevent the error from the process builder.

Thanks,

Bill

Interesting. I will do some research on this. Thanks Bill! It may be over my head as the page is on the Task model and the rendering will have to go through the WhoId and then relate it to an existing Campaign Member on a specific Campaign. Unless I am overthinking it, but guides me in a direction to think it through.

TJ,

What I am thinking that you can do is to add a model to your page that does a query on Campaign Member where the ContactId is the WhoId from your task.  If this model is empty, then display your field.

If this is a page that creates a New Task, then you’ll need to add a model action to do the Campaign Member lookup when the WhoId changes.

Thanks,

Bill