Go: logging lib doesn't notice if log dir doesn't exist

A test program:
```
package main

import (
        "github.com/scionproto/scion/go/lib/log"
)

func main() {
        log.SetupLogFile("test", "/belgium/aka/doesnt/exist", "debug", 0, 0, 0, 0)
        log.Info("Testing")
        log.Flush()
}
```
No errors are output, the program just exits silently with return code `0`.