Summary:
Context: https://fb.workplace.com/groups/edenfswindows/permalink/828914994691047/
Even with D27872753 it doesn't really make sense to have `eden fsck` running on Windows since it requires EdenFS repository to **be unmounted**.
This diff changes it to generate a warning to redirect users to run `eden doctor` instead (which is likely what they need).
Reviewed By: kmancini
Differential Revision: D28203778
fbshipit-source-id: ae105678876903bcf6514252bf07189775f9b187