use gopkg.in/errgo.v1
Merge pull request #8 from rogpeppe/001-use-gopkg-errgo

use gopkg.in/errgo.v1
Revert "use gopkg.in/errgo.v1"
Merge pull request #9 from juju/revert-8-001-use-gopkg-errgo

Revert "use gopkg.in/errgo.v1"
Add a "NotValid" error.
Fix a typo.