Alternate pipeline for Error Handling

startup.cs

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

namespace ErrorHandler3 {
    public class Startup {
        public void Configure (IApplicationBuilder app, IHostingEnvironment env) {
            
            RequestDelegate handler = async (context) => {
                var errorFeature = context.Features.Get<IExceptionHandlerFeature> ();
                await context.Response.WriteAsync ("Error handler : " + Environment.NewLine + errorFeature.Error.Message);
            };

            var errorHandlerOptions = new ExceptionHandlerOptions () {
                ExceptionHandler = handler,
                ExceptionHandlingPath = "/error"
            };

            var subAppBuilder = app.New ();
            Action<IApplicationBuilder>configure = (newApp) => {

                newApp.Run (async (context) => {
                    await context.Response.WriteAsync ("Alternate reality :  " + context.Request.Path);
                });
            };

            var exceptionHandlerPipeline = subAppBuilder.Build ();

            app.UseExceptionHandler (configure);

            app.Use (async (context, next) => {
                if (context.Request.Path == "/fail") {
                    throw new Exception ("Something went wrong !");
                }

                await context.Response.WriteAsync ("No Errors" + Environment.NewLine);
                await next.Invoke ();
            });
            app.Run (async (context) => {
                await context.Response.WriteAsync ("Page : " + context.Request.Path);
            });

        }
    }
}

In this code I'm trying to use Alternative pipepline capability of Error Handling middleware.
When something goes wrong aka Error, Error handling middleware starts a new Action . We can have different pipleline than main pipeline for managing Exeption Handling.