guard against file corruption during download

[boubou19]

Today i tried to assemble manually the 2.2.0.0 RC3 via DAXXL, without changing the locale, resulting in #47. So when i saw that, i terminated my python process, changed the local and rerun DAXXL. That caused the downloads to be killed, resulting in corrupted files.

Because DAXXL assumes that if a file is present, then it's valid, i managed to assemble """valid""" 2.2.0.0 RC3 with a weight of 355MB instead of 435MB.

This is a real issue as if we can assume the cache is corrupted if UnicodeDecodeError arise during download, we have no check against random packet corruption when files are downloaded.

[mitchej123]

Sounds like we need file hashes or size estimates

[mitchej123]

Another option is to start signing release jars.... Daxxl can verify those..... 🤔