services/content: move service client into package

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

services/content/helpers.go

@@ -0,0 +1,19 @@
+package content
+
+import (
+	"github.com/docker/containerd/content"
+	"github.com/pkg/errors"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+)
+
+func rewriteGRPCError(err error) error {
+	switch grpc.Code(errors.Cause(err)) {
+	case codes.AlreadyExists:
+		return content.ErrExists
+	case codes.NotFound:
+		return content.ErrNotFound
+	}
+
+	return err
+}