Fix archive.org fallback URLs for versions with spaces

[HebaruSan]

Problems

• Spaces aren't allowed in archive.org bucket names, and recently a few mods have put spaces in their version strings, which broke the Mirrorer. To fix that, that Don't allow spaces in filenames for archive.org NetKAN-Infra#307 replaced those spaces with underscores. This allowed the mod to be mirrored, but the client doesn't perform the same substitution yet.
• In another branch, I have some code that generates CkanModules with null download properties, and after switching back out of that branch, trying to install such a module threw a null reference exception

Changes

• Now Ckan.InternetArchiveDownload replaces spaces with underscores, so the client will be able to find the fallback downloads of affected modules
• Now the GetInProgressFileName calls are more careful to avoid null references

I think I'll self-review this since it's so minor.