如何显示确切的错误而不是ASP.NET Web API返回UnknowError(500)

<system.web>
  <customErrors mode="Off" />
  <compilation debug="true" />
</system.web>

public class ApiExceptionFilter : IExceptionFilter
{
    public void OnException(HttpActionExecutedContext context)
    {
        if (context.Exception is SomeSpecificException)
        {
            var response = new HttpResponseMessage(HttpStatusCode.InternalServerError)
            {
                Content = new StringContent("具体的错误信息")
            };
            context.Response = response;
        }
    }
}

config.Filters.Add(new ApiExceptionFilter());

public class SpecificException : Exception
{
    public SpecificException(string message) : base(message)
    {
    }
}