Follow-up from "BulkImports: Handle network errors" - handle 50X errors with `BulkImports::NetworkError`
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The following discussion from !68582 should be addressed:
- [ ] @georgekoltsov started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68582#note_681938533): (+2 comments)
> Should we also have a separate list of exceptions for server errors? (50X). Perhaps as a followup.
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