methods return non-nil resources on error

[willnorris]

take for example:

repo, _, err := client.Repositories.Get("foo", "bar")

If foo/bar is not a valid repository, I would expect repo to be nil. Currently, it's set to an empty (but non-nil) Repository. I guess I just didn't really think about this when setting things up initially. This should be fixed throughout the library. It won't require changing any method signatures, just some behavior.