This commit improves error handling in two specific scenarios:
* Missing Download Links: A 5-second delay is introduced when a download
link is missing, as low-level retries aren't enough. Empirically, it
takes about 30s-1m for the link to become available. This resolves
failed integration tests: backend: TestIntegration/FsMkdir/FsPutFiles/
ObjectUpdate, vfs: TestFileReadAtNonZeroLength
* Unrecoverable 500 Errors: The shouldRetry method is updated to skip
retries for 500 errors from "idx.shub.mypikpak.com" indicating "no
record for gcid." These errors are non-recoverable, so retrying is futile.
wiserainandGitHub
f904378c4d