KafkaIO reader should handle runtime exceptions kafka client

KafkaIO reader might stop reading from Kafka without any explicit error message if KafkaConsumer throws a runtime exception while polling for messages. One of the Dataflow customers encountered this issue (see user@ thread)]

'consumerPollThread()' in KafkaIO deliberately avoided catching runtime exceptions. It shoud handle it.. stuff happens at runtime. 

It should result in 'IOException' from start()/advance(). The runners will handle properly reporting and closing the readers.