gateway: return 400 on /ipfs/invalid-cid

[hacdias]

On #185 we unified the behaviour of all resolution errors to return 500. However, this raised the concern that requests to /ipfs/invalid-cid should still return 400. Initially, we decided not to do so since we could not find an easy way to return it and with a unified behaviour, it would be easier for implementations.

TODO

• Create PR here to ensure /ipfs/invalid-cid returns 400 Bad Request. Likely, we have to detect the invalid CID error type somehow. We may want to implement the .Is interface for simplicity. feat(gateway): invalid CID returns 400 Bad Request kubo#9726
• Update ipfs/specs to reflect this change: docs: return 400 Bad Request on Invalid CIDs specs#384