Return 404 from the page tags REST resource when the page doesn't exist

GET /rest/wikis/{wiki}/spaces/{space}/pages/{page}/tags returns 200 with an empty list of tags when the page doesn't exist, while the other REST resources of a page (e.g., the page itself, its objects, its attachments or its history) return 404 for a page that doesn't exist.

The tags resource should also return 404 for a page that doesn't exist, so that clients can distinguish a page without tags from a missing page. As this changes the behavior of the REST API, it should be mentioned in the release notes.