Expand download validation to validate existing of all platforms

[mhdawson]

We seem to fairly regularly get reports like this one (often on ARM)

nodejs/node-v8#19 (comment)

We have a download validation job, but it currently does not fail if a platform is missing.

May be useful to add validation that downloads for all platforms exist.

@gdams since you did most of the work in the download validation script.

[refack]

AFAICT checking nightlies (and canary nightlies) at least, is blocked by #908

[targos]

For canary nightlies, I don't think we should block the download if a platform is missing.

[Trott]

Closing due to long period of inactivity. Feel free to re-open if this is a thing. I'm just trying to close stuff that has been ignored for sufficiently long that it seems likely it's not something we're going to get to.