Return 504 Gateway Timeout with meaningful error on Saturn timeout

[lidel]

Timeout for getting a block from saturn:

https://github.com/ipfs/bifrost-gateway/blob/15477831722cc3fe1f5e64b3ca7d8a755825178b/blockstore.go#L17

Should

• produce 504 Gateway Timeout, and not 400 Bad Request.
• have human-readable error informing about Saturn CDN not returning block

[hacdias]

I am not implementing this right now, but I am leaving here some notes on how to do this:

1. In go-libipfs/gateway:
   a. Create and export a new error ErrGatewayTimeout
   b. If the previous error is returned from any API call, we should errors.Is and return 504.
2. In bifrost-gateway:
   a. Detect "context deadline exceeded" errors from the block store and convert them into gateway.ErrGatewayTimeout.

I think it could also be useful to have 502 Bad Gateway. In filecoin-saturn/caboose#19, we are exposing ErrNoBackend, which effectively has the same meaning as 502 since we could not proxy the request. Alternatively, we could keep returning 500, but I would prefer not to and be more specific.

[lidel]

Related: filecoin-saturn/L1-node#222

bifrost-gateway is using Saturn L1s as gateways.
We should return 504 on timeouts, and 502 on instant failures of these L1s.

Once filecoin-saturn/caboose#28 is done, we should be able to tell when to return 504 (timeouts and 504 codes from remote L1) and when 502 (every other err coming from caboose)

[lidel]

filecoin-saturn/caboose@9b01561 makes errors more useful, they now include status code from L1

[lidel]

Done in #52 which got merged as part of #50