How to throw custom error from Velocity template using lambda - exception

Question

In my Lambda I am throwing custom error but from the vtl I am unable to get that when testing in Postman. In the postman I am keep getting message "A custom error was thrown from a mapping template." but I am expecting to get message and object from the custom class like source and errors
Here is my lambda function and custom error file
Custom Error file
export class ApiError extends Error {
source: string
args: string | undefined
errors: [{[key: string]: string | {[key: string]: string}}] | null | string
constructor(source: string, methodArgs: string, errors: [{[key: string]: string | {[key: string]: string}}] | null | string ) {
super();
this.source = source
this.args = methodArgs
this.errors = errors
}
}
export const completeProcess: Handler = async (event: any): Promise<CustomResponse> => {
const { customerId } = event.arguments.request as CustomRequest
if (customerId === null) {
throw new ApiError("completeProcess", JSON.stringify(event.arguments.request), "customerId missing")
}
}
Vtl file
request.vtl
{
"version": "2018-05-29",
"operation": "Invoke",
"payload": {
"field": "getError",
"arguments": $utils.toJson($context.arguments)
}
}
response.vtl
#if($context.error)
$utils.error($context.result.source, $context.result.errors)
#else
$utils.toJson($context.result)
#end
When running with above files I am keep getting response as
{
"data": {
"completeProcess": null
},
"errors": [
{
"path": [
"completeProcess"
],
"data": null,
"errorType": null,
"errorInfo": null,
"locations": [
{
"line": 2,
"column": 2,
"sourceName": null
}
],
"message": "A custom error was thrown from a mapping template."
}
]
}