Dispatch turbo:load after loading 404 response

[domchristie]

I'm not 100% sure if this is the correct approach, but as far as I can tell, this just dispatches the turbo:load event after rendering a 404.

Fixes #958

[afcapel]

I think this is correct. visitCompleted is basically clearing the busy state and firing the load event, both of which we want on load.

@domchristie happy to merge if you fix the merge conflict.

[domchristie]

@afcapel I suppose the only consideration is if we want to maintain symmetry with complete i.e. call this.delegate.visitFailed(this) (which would then just call this.visitCompleted)

[afcapel]

@domchristie I think it's better if we avoid the indirection and keep the call to this.visitCompleted inside Visit itself.