Improve error message when channel not found

[onnozweers]

I have a use case listening to a channel, where this returns a 404:

curl -X GET "https://dcacheview.grid.surfsara.nl:22880/api/v1/events/channels/z2lDbgy8XaeduITvbubfNg" -H  "accept: text/event-stream"

The body of the reply contains a more descriptive error message:

{"detail":"No such channel","title":"Not Found","status":"404"}

It would be nice to show this "No such channel", because it is much more informative than only a 404.

We have already made such an improvement: see #13