[fix] catch exceptions for file transport unlinkSync

Using cluster, I sometimes run more than one winston logger on a
single instance. Occasionally they both detect that they should
remove the log file, and one will unlink before the other. This
adds a try...catch to make sure that the unlinkSync call doesn't
throw.

I'd be happy to modify the PR to provide some way of notifying the
developer that the unlink failed. I didn't see a good one off the bat,
ordinarily I'd use something like debug().