I think it would be semantically more correct to return a HTTP status code 404 if I'm trying to read a non-existent collection. Now the client gets a 400 client error.

Current behaviour:

> GET /collections/non_existent_collection HTTP/2
> Host: ***
> accept: */*

< HTTP/2 400 
< date: Fri, 24 Sep 2021 11:00:59 GMT
< content-type: application/json
< content-length: 107

According to MDN, I think the 404 code fits perfectly:

The HTTP 404 Not Found client error response code indicates that the server can't find the requested resource.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/404