Do Panic on errors with fsync() and possibly close()

[alexeyklyukin]

We need to do this to avoid silently loosing data. See https://www.postgresql.org/message-id/flat/CAMsr%2BYHh%2B5Oq4xziwwoEfhoTZgr07vdGG%2Bhu%3D1adXx59aTeaoQ%40mail.gmail.com (the same reasons apply to the this application).