api/services/content: define delete method

Allow deletion of content over the GRPC interface. For now, we are going
with a model that conducts reference management outside of the content
store, in the metadata store but this design is valid either way.

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

content/store.go

@@ -81,7 +81,7 @@ func (cs *Store) Delete(dgst digest.Digest) error {
 			return err
 		}
 
-		return nil
+		return errNotFound
 	}
 
 	return nil