Leading space in logstash.yml silently ignores the rest of the file.

A simple document like this, with a mistake of adding leading space:

```yaml
pipeline.workers: 8
queue.type: persisted
```

The leading space of the first entry convinces the YAML parser that all elements must lead with the same space. We can see this in Ruby by doing the following:

```
>> YAML.load(" one: one\ntwo: two\n it doesn't matter what goes here")
=> {"one"=>"one"}
```

No syntax error. No warnings. The remainder of the user input is simply ignored! This is a bad user experience.

Proposed behavior:

* This scenario should be a configuration error and should cause Logstash to fail when processing this yaml.
* When reporting this error, we should provide an actionable message for the human.