Lazy filed backed index error logged after index is deleted 

After deleting an index, we see this log:


```
2023-03-15T23:07:31.832Z ERROR quickwit_metastore::metastore::file_backed_metastore::lazy_file_backed_index: fetch-metadata-error error=IndexDoesNotExist { index_id: "stackoverflow" }
```

The file-backed index metastore is trying to reload the index file, but it's no more here. We could avoid logging this error.
