Don't crash in DownloadHashFile

[vbfox]

When the cache folder didn't exists DownloadHashFile was crashing instead of working as expected.

See #2368 (comment) comment by @matthid for context (Sorry).

[matthid]

No worries :). I'm actually still happy that I don't need to context switch back and forth.

Interstingly though I still see it https://ci.appveyor.com/project/SteffenForkmann/fake/build/1.0.3225#L873.
But I don't think its because of your changes. I think what happens is that msbuild starts all the restore operations at the same time so all the bootstrapper processes check the cache and then start downloading.

I'll try to verify this by invoking the bootstrapper manually in the build process...

[matthid]

That also possibly explains why it couldn't fall back to an existing paket.exe...
Sorry, if I'd have come up with this earlier the fix wouldn't have been urgent at all..
I've logged this here. I guess we need to decide if there is even a good fix for that at all...