Adds a `DiskSpace` error variant and an `is_disk_full()` helper
alongside a `check_disk_space()` util that uses libc's `statsfs` on
unix. FC now detects disk space errors and logs recovery instructions
if applicable.
Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ia6d2c472219dc9d6eed6901dc733d7bb6a6a6964
