Return proper JSON validation errors from API Gateway Response

I'm working on developing a REST Api. I have created a model for one of my resources and am using it to validate a post request. I would like to return any validation errors as a properly formed JSON string.

I have a Gateway Response for Bad Request Body defined like this:

{
    "message": $context.error.messageString, 
    "errors":"$context.error.validationErrorString"
}

This works, but the issue I have is that what is returned by $context.error.validationErrorString is not a valid JSON string. If I submit the request with no information, the response returned returned is

{
  message: 'Invalid request body',
  errors: '[object has missing required properties (["contactEmail","contactMessage","contactName","contactReason","contactSubject"])]'
}

The string returned in the errors property is not valid JSON. If in my calling client I try to parse the error string has JSON it fails to parse.

So, my question is, is there a way with Gateway Responses to get the validation error string as a properly formed JSON string? If not, what is the recommended way to handle this? Why isn't this string a properly formed JSON string since it would seem that its intent is to be returned in a JSON response.

Incidentally, if it matters, this API end point is proxying to a Lambda function, but I want API Gateway to do the validation before the Lambda function is called.