Validate sharded directory structure in `ipfs ls`

Currently, when `ipfs ls` lists a sharded directory, it naively walks the dag without actually validating the internal structure. This means `ipfs ls /ipfs/QmFoo` might list some file named "bar", and `ipfs get /ipfs/QmFoo/bar` might then fail (because the directory was malformed).

We _should_ be validating this structure as we traverse it.