Support '429' responses from Fastly API

[kpfleming]

When the build process is uploading a large number of assets to a KV Store. Fastly's API rate limiting can cause some of the API calls to fail (they receive a 429 response). The response includes information about how long the code should wait before retrying, so it would be good if the Fastly API wrapper in this tool was able to react to that and retry the operation at the appropriate time.

[kpfleming]

While this is in the works, it would be useful to also be able to retry KV store operations which failed due to timeouts or other non-permanent reasons.