Add logging and error handling inside, and outside of FileCommit.
Functions such as fsync, fdatasync will return error in case of hardware I/O errors, and ignoring this means it can silently continue through data corruption.
(c.f. https://lwn.net/SubscriberLink/752063/12b232ab5039efbe/)

EINVAL is handled specially to avoid crashing out on (network, fuse) filesystems that don't handle `f[data]sync`.

I checked that the syncing inside leveldb is already generating an I/O error as appropriate.