Consider some form of policy for deleting files on SHA512 hash check failure.

[BillyONeal]

As requested by @strega-nil in #15474 , if the user gets a bad download, they get "stuck" and need to manually delete the file. We probably don't want to simply delete the file on mismatch because there might be other things touching it, but we probably can do better than our current throw up our hands and give up strategy.

Note that this issue applies to all places vcpkg downloads things, including boostrap.sh, vcpkg_download_distfile, vcpkg fetch, etc.