Import/Export fails to export projects due to errors while exporting merge requests

Summary

In Kibana, there are occurrences of project exports that failed due to missing merge request diff files in the disk (Errno::ENOENT) or in the object store (Gitlab::HttpIO::FailedToGetChunkError). And with error connection to object-store (SocketError).

Relevant logs and/or screenshots

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Because the same code is used by GitLab Migration, this issue might also occur when project export is enabled on GitLab Migration

KQL: json.extra.sidekiq.class : "ProjectExportWorker" AND json.exception.backtrace : "app/models/merge_request_diff.rb"

Kibana - Internal only

Relevant code lines

https://gitlab.com/gitlab-org/gitlab/-/blob/acb1e01622ef96e6d579aef7ad9cf1634514a008/app/models/merge_request_diff_file.rb#L17-28

https://gitlab.com/gitlab-org/gitlab/-/blob/acb1e01622ef96e6d579aef7ad9cf1634514a008/app/models/merge_request_diff_file.rb#L60-88

Iterations 👣

1. For the first iteration - this issue, we will change the code not to halt the whole project export process. So we will rescue the exception, return a blank diff, log that the diff was skipped, and continue the export process.
2. For the next iteration - #371602, we will improve the user experience by implementing a way for them to know the merge request diff was skipped.