Change raise of exception which does not exist

This changes a use of ``DockerError`` (which does not exist) to ``DockerException``.

docker/models/plugins.py

@@ -113,7 +113,7 @@ def upgrade(self, remote=None):
                 A generator streaming the decoded API logs
         """
         if self.enabled:
-            raise errors.DockerError(
+            raise errors.DockerException(
                 'Plugin must be disabled before upgrading.'
             )