Download script improvements

[akx]

This PR:

• makes sure the download script doesn't clobber any existing files if they seem correct enough (same size as remote)
• ensures 404, etc. errors don't get written as outputs (by raising an error when they occur)
• uses a larger read chunk size for a modest speed boost; there's no need to read and write things 1000 bytes at a time

[WuTheFWasThat]

sorry for getting to this so late, could you resolve the conflicts and i'll take another look?

[akx]

@WuTheFWasThat Rebased!