Use error interfaces for content/metadata

These interfaces allow us to preserve both the checking of error "cause"
as well as messages returned from the gRPC API so that the client gets
full error reason instead of a default "metadata: not found" in the case
of a missing image.

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

remotes/docker/pusher.go

@@ -31,7 +31,7 @@ func (p dockerPusher) Push(ctx context.Context, desc ocispec.Descriptor) (conten
 	status, err := p.tracker.GetStatus(ref)
 	if err == nil {
 		if status.Offset == status.Total {
-			return nil, content.ErrExists
+			return nil, content.ErrExists("")
 		}
 		// TODO: Handle incomplete status
 	} else if !content.IsNotFound(err) {
@@ -72,7 +72,7 @@ func (p dockerPusher) Push(ctx context.Context, desc ocispec.Descriptor) (conten
 					// TODO: Set updated time?
 				},
 			})
-			return nil, content.ErrExists
+			return nil, content.ErrExists("")
 		}
 		if resp.StatusCode != http.StatusNotFound {
 			// TODO: log error

remotes/docker/status.go

@@ -34,7 +34,7 @@ func (t *memoryStatusTracker) GetStatus(ref string) (Status, error) {
 	defer t.m.Unlock()
 	status, ok := t.statuses[ref]
 	if !ok {
-		return Status{}, content.ErrNotFound
+		return Status{}, content.ErrNotFound("")
 	}
 	return status, nil
 }