Merge pull request #384 from ipfs/libipfs/issues/200

docs: return 400 Bad Request on Invalid CIDs

http-gateways/PATH_GATEWAY.md

@@ -679,8 +679,9 @@ Gateway MUST respond with HTTP error when it is not possible to traverse the req
 - [`404 Not Found`](#404-not-found) should be returned when the root CID is valid and traversable, but
 the DAG it represents does not include content path remainder.
   - Error response body should indicate which part of immutable content path (`/ipfs/{cid}/path/to/file`) is missing
+- [`400 Bad Request`](#400-bad-request) should be returned when the root CID under the `ipfs` namespace is invalid.
 - [`500 Internal Server Error`](#500-internal-server-error) can be used for remaining traversal errors,
-such as invalid CIDs, domains that cannot be resolved, or IPNS keys that cannot be resolved.
+such as domains that cannot be resolved, or IPNS keys that cannot be resolved.
 
 ## Best practices for HTTP caching