Provide more details around "Trouble parsing json"

This error is really difficult to work with as it provides no context;

```
{:timestamp=>"2015-06-09T17:42:11.803000-0700", :message=>"Trouble parsing json", :source=>"message", :raw=>"", :exception=>#, :level=>:warn}
```

It'd be good if we logged the message that generates this as otherwise we have no way of dealing with it other than adding an `output { stdout { codec => rubydebug } }` to high traffic level systems just to catch the occasional error.

Start by reproducing the “Trouble parsing json” warning using the empty raw message shown in the report, then trace the parser-error path that emits it. Use the reported event and exception as the acceptance criteria; done means the warning includes the message that caused the parsing failure without requiring an additional stdout rubydebug output.

Written by the indexing model from the issue text.