Replace uses of log.Fatal with log.Panic

[Sneagan]

          I don't like this `Fatal` here at all (because nothing deferred will happen). But there are already quite a few others.

Technically, this is incorrect. The opened file won't be properly closed. If you want to terminate without too much hassle in this closure, better use log.Panic instead.

Originally posted by @pavelbrm in #2544 (comment)