Raster tile server responds badly to non-existent tile

[davenquinn]

It appears that when we request a raster tile that is outside the bounds of the queried raster image, we return a 502 status code, when it should be a 404 (probably)?

https://tileserver.development.svc.macrostrat.org/cog/tiles/10/193/388.png?url=https://s3.macrostrat.chtc.io/map-datastore/Provo30x60_OFR-586DM.tif

This wreaks havoc with certain clients, especially the Mapboard app, which goes into a cascade of request retries.

We should also store raster bounding boxes to do a better job of tiling together imagery. We may be getting 502s because the tileserver only knows that it can't read the requested file, not that it actually doesn't exist.