Doesn't work when with virtualbox shared volumes

Basically, we try to sync a directory, windows fails with "invalid argument" ~(windows doesn't support syncing directories) and vbox forwards this error~.

~We actually have a check to avoid syncing directories on windows. Unfortunately... in this case, we think we're on Linux. I believe the correct fix is to detect that the error is an "invalid argument" error and ignore it (we can't sync anyways, nothing really failed).~

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Edit: It turns out that vboxsf just doesn't support fsync on directories at all. Really, we probably should just be ignoring fsync errors on directories entirely (possibly logging them?). There's not much we can do anyways.