Smarter ErrNotFound

[whyrusleeping]

We should switch all of our 'not found' style errors over to the go style of having typed errors. Where every 'not found' error has a NotFound() bool method on it, so that it can be type checked easily. This will fix tons of fragile if err == bstore.ErrNotFound type calls.

The main question here is whether or not to add a IsNotFoundErr(err) bool method in some help package, or reimplement as needed (its a one-liner). One thought I had here was that we could put this in our own custom errors package, (a wrapper around the pkg/errors package) and have that be used everywhere.

[Stebalien]

Having a common error package would be really nice.