refactor(blockservice, merkledag, namesys) deprecate u.ErrNotFound

In a couple places we have the choice between returning an exported error and returning an error with information. Which do we prefer? @jbenet @whyrusleeping 
https://github.com/jbenet/go-ipfs/commit/0bfadea0366a7997ddb2d6200124a8edbdc6d3cc