Retry transmissions after cURL error

[tohvanah]

I'm managing a site sending about 30 emails a day. Lately there has been about one email a day that fails with a cURL error (see below). I believe under these circumstances, and possibly others ( #137 ), a retry should be attempted. I've considering programming one myself using the wpsp_handle_response hook.

cURL error:
{"errors":{"http_request_failed":["cURL error 28: Operation timed out after 15001 milliseconds with 0 bytes received"]},"error_data":[]}