logstash 2.0 doesn't log failure to create new index as a result of mapping error

Moving this from https://github.com/elastic/logstash/issues/4475

> there has been a report that if an index request that results in a creation of an index fails due to a mapping conflict with the template no error is logged and logstash discards the event.
> this was reported in 2.0 but hasn't been confirmed or checked if fixed in recent 2.x
