Failure to delete model when model is training should not return 200

[jmazanec15]

Currently, when we try to delete a model while it is training, we block the request:

curl -X DELETE "localhost:9200/_plugins/_knn/models/my-model3?pretty"
{
  "model_id" : "my-model3",
  "result" : "failed",
  "error" : "Cannot delete model \"my-model3\". Model is still in training"
}

However, this request returns a 200 response code, even though it failed to delete the resource.

curl -I -X DELETE "localhost:9200/_plugins/_knn/models/my-model3"
HTTP/1.1 200 OK

I believe that this should throw a 409 conflict error code.

[navneet1v]

Closing this issue as the PR is merged.