[Featureserver] Handling of 'record not found'

[Peter Robins]

AFAICS, there's no check at the moment that a fid in the url exists
(at least in the datasources I've looked at). If I try to read, update
or delete a fid that does not exist, I get a python error, but with a
200 response code. So, if I'm using this for example with OL
protocol/strategy, OL interprets this as meaning 'transaction
successful', which it wasn't.

Shouldn't FS be returning 404 code if the fid in the url doesn't exist?