Display an error message when LFS object is missing
This is a follow up issue from the [comment](https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/4903#note_2422439880+)
Currently, if an LFS object is missing in a merge request, the content is not displayed for security reasons. However, nothing is shown at all — it would be better to display a proper error message, as suggested here:

issue