Display source of error message in logs

Error messages coming from plugins (input, filter, output) often don't display any source. In some cases, it's pretty obvious where it failed but a lot of the time, it takes some time to figure out where it's coming from.

It would be nice if Logstash added the source plugin name at the beginning of each error message (such as 'filter/ruby: ') to help locating where the issue is coming from.

Here are some example of unclear messages:
- logstash-plugins/logstash-filter-ruby#14
- logstash-plugins/logstash-filter-geoip#51
- logstash-plugins/logstash-output-elasticsearch#294
- logstash-plugins/logstash-input-unix#15
- logstash-plugins/logstash-input-rabbitmq#48
- logstash-plugins/logstash-output-elasticsearch#300