Handle 404 errors gracefully when viewing deleted entities in management UI

[kickster97]

Describe the bug If you have an exchange view open and the exchange is deleted you'll get a "Something went wrong: Error fetching data: [object Object]" error above the bindings table. We're probably having similar messages in other views.

How to reproduce Open the management UI and have the exchange view open, while an exchange gets deleted via amqp.

Expected behavior We should handle 404s better and display that the entity is (probably) deleted.