When a DSO doesn't exist, we should show the 404 component.

[artlowel]

Describe the bug
If the id for a DSO can't be found you don't see the 404 component, instead the page will be broken.

To Reproduce
Steps to reproduce the behavior:

1. enter the url of a community, collection or item page with a UUID that doesn't exist

Expected behavior
You should see the 404 component. The URL should still be the one you entered.

I expect this will take 4h to fix.

[artlowel]

The exception suggested here could be a clean way of handling this issue, however, it is unclear whether it is something that works right now, or is a suggestion for the angular devs: angular/angular#29089 (comment)