use validation metadata from other servers to validate this server's data?

[zooko]

The Merkle Tree over the shares is stored on each of the N servers. If a server has the correct share, but its copy of the Merkle Tree over the shares has been corrupted, then the downloader could download the share and check it using a copy of the Merkle Tree which was downloaded from a different server.

However, currently the downloader stops using a server entirely if it has a corrupted Merkle Tree, so the downloader might fail to download the file since it doesn't try to use the share from the server that has the corrupted Merkle Tree.