Cater for 503 and 429 http codes when downloading.
Cater for 503 and 429 http codes when downloading.

429 may be sent in the future and results in backing off before retrying
503 is already sent and we need to cope with it when the raid part is not currently available on that server - in which case switch to another part to continue the download
503 for normal files should just back off slightly and retry.

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