refactor(blockservice) export blockservice.ErrNotFound
refactor(merkledag) use one-off error
refactor(namesys) use one-off error
fix(namesys, merkledag) use static error
fix(namesys) use the error that already exists
fix(merkledag) return static error
Merge pull request #224 from jbenet/issue-209-plus

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