better way of handling get tile errors

[reyemtm]

Refer to mapbox/mapbox-gl-js#1800 -

Need better way to handle errors when no tiles are present. This one is beyond me, but for the moment I am just sending

res.status(204)

instead of the 404 error, but there must be a better way. Maybe when mapbox figures it out we can roll that into this server.