question

Well first, InsufficientFunds is only returned from a purchase operation, which is called from the client. I believe what you're saying is that you want to do something more complex than a simple purchase (like requiring multiple VCs for the "purchase", or VC+item instance, for example), and that you want to return this error, so with that as the assumption:

There are two schools of thought on this, both of which are valid - it just depends on how you want to manage your title. In both cases, it's important to understand that you're distinguishing the error which occurred in an API call in the script from an error of the script itself running. I realize this may sound like splitting hairs, but it's an important distinction. In terms of how you present the error details to the client code the two recommendations would be:

1. If you do nothing else, any Server API call made in a Cloud Script that receives an error response causes an exception to throw. That will result in all the details of the error being in the response, like this:

{
    "code": 200,
    "status": "OK",
    "data": {
        "FunctionName": "exampleAPIBadRequest",
        "Revision": 1,
        "Logs": [{
            "Level": "Error",
            "Message": "PlayFab API request error",
            "Data": {
                "api": "/Server/UpdateUserStatistics",
                "request": {
                    "PlayFabId": "ADADCB6F9583EA99"
                },
                "result": null,
                "apiError": {
                    "code": 400,
                    "status": "BadRequest",
                    "error": "InvalidUserStatistics",
                    "errorCode": 1073,
                    "errorMessage": "Invalid user statistics",
                    "errorHash": null,
                    "errorDetails": null
                }
            }
        }],
        "ExecutionTimeSeconds": 0.0754191,
        "ProcessorTimeSeconds": 0.016,
        "MemoryConsumedBytes": 122968,
        "APIRequestsIssued": 1,
        "HttpRequestsIssued": 0,
        "Error": {
            "Error": "CloudScriptAPIRequestError",
            "Message": "The script called a PlayFab API, which returned an error. See the Error logs for details.",
            "StackTrace": "Error\n    at handlers.exampleAPIBadRequest (5F4-main.js:3:12)"
        }
    }
}

Note that in this case, the response code is a 200 (status OK), as the execution of the script completed - it's the logic within the script that had a failure, and so is being reported on.

2. Alternately, you can manage any errors or exceptions in the script using try/throw/catch, and compose your own custom FunctionResult. This would be the way to go if you want to have server-authoritative fallback logic in the script itself.