Make neuronlistfh/remotesync downloads more robust

[jefferis]

@Robbie1977 and Asa Barth-Maron have both run into sporadic download issues with large neuronlists. It would be good to offer some kind of retry option and perhaps only do a hard fail if the number of retries seems excessive. It might also make sense to do a first pass and then retry any neurons that are still missing.