Show "Event not found" when an event is not available

[raimund-schluessler]

When trying to open an event which doesn't exist (simply edit the string in the address bar to something random), the app reloads and then shows a kind of broken sidebar:

Instead of showing such default content and an everlasting spinner, I think it would be good to show that the event was not found. Otherwise the user might think something broke. This should be easily possible once nextcloud-libraries/nextcloud-vue#1281 is in.