WebApi2 - Retrieve ErrorMessage From DataAnnotations

I want to return the custom error message that I have specified in my data model class to return to user.  I've tried creating a filter for override OnActionExecuting(HttpActionContext) but I am still unable to access the custom message.  Am I going about this the wrong way or missing something else?

//In data model:
public class LogsDto 
{
        [Display(Name = "ip")]
        [Required(ErrorMessage = "IP Address is invalid.")]
        [DataMember]
        public string ip { get; set; }
}


//In WebApiConfig.cs:
config.Filters.Add(new ValidateModelAttribute());

//In OnActionExecuting:
public override void OnActionExecuting(HttpActionContext context)
{
if (!context.ModelState.IsValid)
{
    //have tried a variety of methods to locate errormessage

    context.Response = context.Request.CreateErrorResponse(HttpStatusCode.BadRequest, context.ModelState);
}

//In Controller:
public IHttpActionResult Post([FromBody()]LogsDto request)
{     
    try
{
    if (!ModelState.IsValid)
        return BadRequest();
    else
    { //do stuff }
}

I can get a result like below but when I dig down through the (HttpActionContext context), the ErrorMessage is empty and that is what I need.

{"Message": "The request is invalid.","ModelState": {"request.ip": ["Unexpected character encountered while parsing value: ,. Path ip, line 2, position 12.","After parsing a value an unexpected character was encountered: \". Path ip, line 3, position 2."
    ]
  }
}