During initial sync, we may receive some error codes which are not network errors but should not cause an initial sync to fail permanently. Specifically, ShutdownInProgress (the sync source may restart normally), PrimarySteppedDown, ExceededTimeLimit, InterruptedAtShutdown, InterruptedDueToReplStateChange, and NotMasterOrSecondary. These (plus NotMasterNoSlaveOK and NotMaster, which we should not receive) are covered under the isRetriableError label.

The catch is that NotMasterOrSecondary may be a permanent error. It indicates a sync source in startup, recovery, rollback, or removed. If it is REMOVED, the error is permanent and we should not retry.