Add error return to Repository method on Registry

The method (Registry).Repository may now return an error. This is too allow
certain implementationt to validate the name or opt to not return a repository
under certain conditions.

In conjunction with this change, error declarations have been moved into a
single file in the distribution package. Several error declarations that had
remained in the storage package have been moved into distribution, as well. The
declarations for Layer and LayerUpload have also been moved into the main
registry file, as a result.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/revisionstore.go

@@ -5,6 +5,7 @@ import (
 	"path"
 
 	"github.com/Sirupsen/logrus"
+	"github.com/docker/distribution"
 	"github.com/docker/distribution/digest"
 	"github.com/docker/distribution/manifest"
 	"github.com/docker/libtrust"
@@ -40,7 +41,7 @@ func (rs *revisionStore) get(revision digest.Digest) (*manifest.SignedManifest,
 	if exists, err := rs.exists(revision); err != nil {
 		return nil, err
 	} else if !exists {
-		return nil, ErrUnknownManifestRevision{
+		return nil, distribution.ErrUnknownManifestRevision{
 			Name:     rs.Name(),
 			Revision: revision,
 		}