Remove duplicate NotFoundError and rename it to ErrNotFound (idiomatic)
Remove duplicate NotFoundError and rename it to ErrNotFound (idiomatic)

Error had to be defined in its own package to avoid import cycles.

klaidliadon/go-git,alcortesm/go-git,alcortesm/go-git,mcuadros/go-git,src-d/go-git,go-git/go-git,ivanfoo/go-git,ivanfoo/go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,src-d/go-git,go-git/go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,mcuadros/go-git