Added missing error page in UI project

Author: Tiago Brenck

Microsoft.Identity.Web.UI/Areas/MicrosoftIdentity/Pages/Account/Error.cshtml

@@ -0,0 +1,23 @@
+@page
+@model Microsoft.Identity.Web.UI.Areas.MicrosoftIdentity.Pages.Account.ErrorModel
+@{
+    ViewData["Title"] = "Error";
+}
+
+<h1 class="text-danger">Error.</h1>
+<h2 class="text-danger">An error occurred while processing your request.</h2>
+
+@if (Model.ShowRequestId)
+{
+    <p>
+        <strong>Request ID:</strong> <code>@Model.RequestId</code>
+    </p>
+}
+
+<h3>Development Mode</h3>
+<p>
+    Swapping to <strong>Development</strong> environment will display more detailed information about the error that occurred.
+</p>
+<p>
+    <strong>Development environment should not be enabled in deployed applications</strong>, as it can result in sensitive information from exceptions being displayed to end users. For local debugging, development environment can be enabled by setting the <strong>ASPNETCORE_ENVIRONMENT</strong> environment variable to <strong>Development</strong>, and restarting the application.
+</p>

Microsoft.Identity.Web.UI/Areas/MicrosoftIdentity/Pages/Account/Error.cshtml.cs

@@ -0,0 +1,33 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using System.Diagnostics;
+
+namespace Microsoft.Identity.Web.UI.Areas.MicrosoftIdentity.Pages.Account
+{
+    [AllowAnonymous]
+    [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
+    public class ErrorModel : PageModel
+    {
+        /// <summary>
+        /// This API supports infrastructure and is not intended to be used
+        /// directly from your code.This API may change or be removed in future releases
+        /// </summary>
+        public string RequestId { get; set; }
+
+        /// <summary>
+        /// This API supports infrastructure and is not intended to be used
+        /// directly from your code.This API may change or be removed in future releases
+        /// </summary>
+        public bool ShowRequestId => !string.IsNullOrEmpty(RequestId);
+
+        /// <summary>
+        /// This API supports infrastructure and is not intended to be used
+        /// directly from your code.This API may change or be removed in future releases
+        /// </summary>
+        public void OnGet()
+        {
+            RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier;
+        }
+    }
+}