JAR cache does not retry downloading after error, user must manually run application again

Hi,

I'm working with project witch use IceTea-web and found one problem.
Sometimes when between client and server is slow unstable connection there is problem with downloading jars to cache (in our case downloading is just interrupted and downloaded file is incomplete). User must run application many times to download all jars.

I think it is because this change :
https://github.com/AdoptOpenJDK/IcedTea-Web/pull/803/files
and this :
https://github.com/karakun/OpenWebStart/issues/401

So user run application. Jars are downloaded to cache and when error occurs, corrupted jar is deleted and user must run application once again.

Could we have some kind of retry mechanism maybe configured in icetea-web properties (retry on/off, retry times)
So when jar is not downloaded properly try to download it once again X times
I think logic for that is in `BaseResourceDownloader.tryDownloading` method