chifra init

[tjayrush]

1. It should report that it failed if it does not complete totally. In fact, it doesn't do that.

2. It should check that the file sizes expected are in fact stored on disc. As it is, it allows partial files to be downloaded.

3. It should allow a --first_block option and not download earlier than that block.

4. instead of collecting together failed files, if a download fails, repeat the attempt to download right away. Otherwise, we get this weird, gapped-filled download if the user quits after some files fail and have not yet been tried again.