fixes #10 by catching exception

This fixes #10 but it still generates a warning:

```
record_reformer: NoMethodError undefined method `[]' for nil:NilClass /Users/marshall/code/random/fluent-plugin-record-reformer/lib/fluent/plugin/out_record_reformer.rb:190:in `block in expand'
```

I'm not sure what the best way to handle this is, this is an improvement because it does not stop it from processing the message, which I think is better than the current behavior in which it will stop processing. 

Since we have several different message formats we do not want to stop processing. 
what's the point of returning `str`? to me it seems a little misguided because it wasn't able to interpolate the value. that's going to emit an event that looks like `"${nothing[\"doesnotexist\"]}", 1412729201, {"doesnotexist"=>"foo"}`. is that what you mean?