[SPARK-6431][Streaming][Kafka] Error message for partition metadata requ...

...ests

The original reported problem was misdiagnosed; the topic just didn't exist yet.  Agreed upon solution was to improve error handling / message

Author: cody koeninger <cody@koeninger.org>

Closes #5454 from koeninger/spark-6431-master and squashes the following commits:

44300f8 [cody koeninger] [SPARK-6431][Streaming][Kafka] Error message for partition metadata requests