Add retry option to download command

[wsnoble]

I am finding that downloading from PRIDE is pretty flakey, and the download command fails in about 10-25% of the runs. I have written my own wrapper code to detect this, wait for 60 seconds, and then retry the download (up to n time). I think you might want to consider adding this functionality to ppx.

[wfondrie]

I've experienced this problem as well. Currently, ppx will already retries connecting up to 10 times, although there is no delay between the attempts.

I'm not sure how helpful it would be to expose this parameter, but it might be helpful to add a delay between the attempts. What do you think Bill?

[wsnoble]

I think adding a delay is worth doing.

I didn't know it was already retrying 10 times. Is this shown in the progress bar in some way? It seems like user-level indication of this behavior would be helpful.