Return 404 for deleted events

[MacsDickinson]

We're getting a fair few sentry errors from the Google spider when it trys to crawl deleted pages. These should return 404 as errors give negative SEO

[MacsDickinson]

Example of the issue on sentry

https://app.getsentry.com/yorkshire-digital/yorkshire-digital-website/issues/113733443/

NullReferenceException: Object reference not set to an instance of an object.
  Module "Nancy.Routing.Route+<>c__DisplayClass4", line 81, in <Wrap>b__3
    System.Threading.Tasks.Task`1[System.Object] <Wrap>b__3(System.Object, System.Threading.CancellationToken)
  ?, in CallSite.Target
    System.Object CallSite.Target(System.Runtime.CompilerServices.Closure, System.Runtime.CompilerServices.CallSite, System.Func`2[System.Object,System.Object], System.Object)
  Module "YorkshireDigital.Web.Events.Modules.EventModule+<>c__DisplayClass0_0", line 449, in <.ctor>b__0
    System.Object <.ctor>b__0(System.Object)
  Module "YorkshireDigital.Web.Events.ViewModels.EventDetailsModel", line 6, in .ctor
    Void .ctor(YorkshireDigital.Data.Domain.Events.Event)