Change ERROR to WARN if no log config found

If you run Logstash manually and point it to a config file, eg if I only want a single run through, you will get this error;
```
$ ~/logstash-5.5.2/bin/logstash -f configfile.conf
ERROR StatusLogger No log4j2 configuration file found. Using default configuration: logging only errors to the console.
```

However the process still runs. This is a little confusing because as a user I'd expect the program to halt if there was an error, rather than continue, just like if there was a syntax error somewhere.

Is it possible to change this error to a warning to make it clearer that this is not ideal, but you can still proceed?