download retry&resume

[ibilon]

Some files like the android ndk are huge and may fail during the download.

The tools should retry the download, maybe behind a prompt ("Try again [y/n/a]"),
and of course not from the beginning.

Note: see haxelib's resume implementation https://github.com/HaxeFoundation/haxelib/blob/development/src/haxelib/client/Main.hx#L745-L774

[jgranick]

Let's track it here:

#878

Ultimately, I think we need a GUI and other improvements (or we need to scrap platform setup)