ChunkManager can contain chunks that refer to shards that were removed

After a shard has been removed, the ShardRegistry can refresh itself and remove the shard. However, it is possible that the ChunkManager to continue on having chunks that refer to the removed shard and get a ShardNotFound error.

One example is the validate command. It uses the ChunkManager to get the list of shards to run the command to, and it will get ShardNotFound error before it even tries to send it.