Fix possible runtime exception

I've tried [Linter](https://github.com/HairyFotr/linter) on scala-ssh and found and fixed this possible runtime exception:

    [warn] /home/hairy/dev/others/scala-ssh/src/main/scala/com/decodified/scalassh/HostConfig.scala:164: This string format will fail with: Conversion = '''
    [warn]           catch { case e: IOException => Left("Could not read host file '%' due to %s".format(file, e)) }
    [warn]                                               ^