refactor: Storage driver errors

Small refactoring of storagedriver errors.
We change the Enclosed field to Detail and make sure
Errors get properly serialized to JSON.
We also add tests.

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

registry/storage/driver/base/base.go

@@ -79,12 +79,10 @@ func (base *Base) setDriverName(e error) error {
 		actual.DriverName = base.StorageDriver.Name()
 		return actual
 	default:
-		storageError := storagedriver.Error{
+		return storagedriver.Error{
 			DriverName: base.StorageDriver.Name(),
-			Enclosed:   e,
+			Detail:     e,
 		}
-
-		return storageError
 	}
 }