How to debug Server-Side JavaScript Activity

I would put your code on a landing page, and test it that way. Like so:

%%[ 
    VAR @Result
]%%

<script  runat="server">

    Platform.Load("core","1");

    try {

    var caslDE = DataExtension.Init("6C119FFD-0EF5-476A-8127-E2E70D80ED04");

    caslDE.Rows.Update({Source:"EXACT"}, ["Prospect Type"], ["A"]);

    } catch (ex) {
        Write("An error has occurred: " + Stringify(ex));
        Variable.SetValue("@Result", Stringify(ex));  //sets above ampscript variable
    }

</script>
//// some basic page html goes here/////
%%=v(@Result)=%%