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. 
I am thinking as:

* If tag becomes `nil` (failed to expand), the message should not be emitted. 
* If some fields become `nil` (failed to expand), the message can be emitted with nil value. 

In both case, we should log.warn. 

The reason why a message with `nil` tag should not be emitted is, fluentd and all plugins consider as tag is not nil always. Emitting `nil` tag probably will make fluentd be broken.