Update WindowedDStream.scala

update the content of Exception when windowDuration is not multiple of parent.slideDuration

Author: baishuo(白硕) <vc_java@hotmail.com>

Closes #390 from baishuo/windowdstream and squashes the following commits:

533c968 [baishuo(白硕)] Update WindowedDStream.scala

Conflicts:

	streaming/src/main/scala/org/apache/spark/streaming/dstream/WindowedDStream.scala
Small syntax error from previous backport