/identities/:id should return 404 when identity doesn't exist

[bjoerge]

It currently gives status code 200 and request body {}.

Any fix for this issue should be done carefully as there may be a lot of code depending on the current status code and response body.

A likely consequence of fixing this issue, xhr success callbacks in JavaScript will never be called and needs to be used as error callbacks instead.

Also requests done through Pebblebed::Http.perform will need to be wrapped in begin/rescue.

I guess its an open question whether this bug is worth fixing. At least in v1.