/delete when unauthorized returns 404 rather than 401 or 403

[dchoi27]

We detail potential responses for /delete here: https://nft.storage/api-docs/

However, we don't see any 401s or 403s returned in practice, just 404s for 4xx errors. When I submit a random unauthorized request (e.g. https://api.nft.storage/delete/asfwaefawef), I get back a 404.

We should either change the API or the docs to reflect this.