Logstash gives confusing error messages if the "logs" path is a file, not a directory.

When ./logs exists but is a file, logstash will emit warnings/errors on startup:

```
ERROR Unable to create file /Users/tanya/logstash-6.0.0-beta1/logs/logstash-plain.log java.io.IOException: Not a directory at java.io.UnixFileSystem.createFileExclusively(Native Method)
```

We should inform the user with a more actionable error message if the path is invalid (ie; 'logs' is a file, not a directory)