Updated UseExceptionHandler() to use the new ErrorController

With the introduction of the `ErrorController` (e7fe64f, bf46877), we now have a valid target for the `UseExceptionHandler()` call. Previously, this pointed to a non-existent page. This was fine for sample purposes, but now that we have an actual page to us, this is preferred.

This could alternatively point to any arbitrary `Topic` under `Error` (e.g., `Error:InternalServer`).

Author: JeremyCaney

OnTopic.AspNetCore.Mvc.Host/Program.cs

@@ -52,8 +52,8 @@
   app.UseDeveloperExceptionPage();
 }
 else {
-  app.UseExceptionHandler("/Home/Error");
   app.UseStatusCodePagesWithReExecute("/Error/{0}");
+  app.UseExceptionHandler("/Error/500/");
   // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
   app.UseHsts();
 }