Return 404 for missing or unauthorised folders

This branch updates the mailbox API to return a 404 response for requests involving missing or unauthorised folders. This enables the client to move on to the next API call without retrying.

As before, the same response is returned for missing folders and unauthorised folders so that the API can't be used to probe for the existence of folders the client isn't authorised to access.

Note: This should be merged on the same day as briar!1692 (merged) to avoid breaking the nightly integration tests.