Startup.cs

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
  app.UseExceptionHandler(errorApp =>
  {
    errorApp.Run(async context =>
    {
      var exceptionHandlingFeature = context.Features.Get<IExceptionHandlerPathFeature>();

      if (exceptionHandlingFeature?.Error is GuardException ex)
      {
        context.Response.StatusCode = (int)HttpStatusCode.BadRequest;
        context.Response.ContentType = "application/json";

        await context.Response.WriteAsync($"\"{ex.ArgumentName}\": \"{ex.Message}\"");
      }
    });
  });
}