gateway: ipld.ErrNotFound should result in a 404

[MichaelMure]

It's a somewhat rare case, but it's possible for a gateway backend to return ipld.ErrNotFound:

• using an offline exchange, for example with --offline in kubo
• possibly https://github.com/ipfs/kubo/blob/master/docs/config.md#gatewaynofetch
• implementing some form of denylist or restriction at the storage/exchange level (which we do at Infura)

In those situations, kubo v0.22.0 / boxo v0.11.0 would return a 404 in the gateway, but the implementation rework changed that to return a 500.

See also https://github.com/ipfs/boxo/pull/440/files#r1297472794

I think 404 is the correct answer, as this is not a timeout while looking for content on the p2p network, or some missing link in an IPLD node (which would mean an incorrect request). It's a definitive answer that the content does not exist.

Optionally, another status code could be returned in some more specific situations (see #591), but 404 seem to be the correct default to me.

MichaelMure@d088e1e trivially change that returned status, with no other impact as far as I can tell.