Retry exceptions during BulkImports::FileDownloadService

What does this MR do and why?

This MR updates BulkImports::FileDownloadService to raise BulkImports::NetworkError when 50* errors occur, which will be retried during a Direct Transfer migration. This is useful, since in a scenario where source Gitlab instance becomes unavailable, we wait and retry in hopes it becomes available later. There are going to be 10 retries with 30 seconds interval in between each retry.

Mentions #432553 (closed)

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