File deletion resilience improvements

`paket install` after `paket convert-from-nuget` on the hyprlinkr project was failing consistently for me.

My env is a stone age desktop on Win 8.1 (with an SSD, but that's beside the point - there's plenty info re it being possible for random delays in files and/or dirs becoming unencumbered and ready to delete for various real and imagined reasons)

The changes here are a minimal fix to the issue.

At one point, Heisenberg effects from logging I added was making it appear there was a strong case for introducing a retry loop. I can no longer repro that, so it's in http://fssnip.net/o0 for the moment 

I sadly can't think of a way to fuzz test this for OS delays without adding a significant perf tax to the test suite -> is that a lack of imagination failure?

One minor observation is that if you supply the following file list:

    A:\a\b\c\d.fs
    A:\b.fs

The `A:\b` dir will remain. If it was desired to remove this issue, that would involve passing in a base dir so I've left that -> lemmeknow if you want it addressed