ASP.NET Core MVC - Developer Exception Page

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace KudvenkatCourse
{
    public class Startup
    {
        private IConfiguration _config;
        public Startup(IConfiguration config)
        {
            _config = config;
        }
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();

                app.UseWelcomePage("/IIS");
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
            }

            FileServerOptions dfo = new FileServerOptions();
            dfo.DefaultFilesOptions.DefaultFileNames.Clear();
            dfo.DefaultFilesOptions.DefaultFileNames.Add("foo.html");
            app.UseFileServer(dfo);
            //app.UseFileServer();
            //app.UseStaticFiles();

            app.UseRouting();
            

            app.Use(async (context, next) =>
            {
                await context.Response.WriteAsync("Hello from .Use!\n");                
                await next();

            });            

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapGet("/", async context =>
                {                    
                    await context.Response.WriteAsync("Hello from .UseEndpoints!\n");
                    await context.Response
                    .WriteAsync(System.Diagnostics.Process.GetCurrentProcess().ProcessName);
                    await context.Response.WriteAsync("\n" + _config["MyKey"]);
                });
            });

            app.Use(async (context, next) =>
            {
                await context.Response.WriteAsync("Hello from .Use 2nd!\n");
                await next();
            });


            app.Run(async (context) =>
            {
                throw new Exception("Holy makarony!");
                await context.Response.WriteAsync("Hello from .Run\n"); 
            });
        }
    }
}