[SPARK-1901] worker should make sure executor has exited before updating executor's info

https://issues.apache.org/jira/browse/SPARK-1901

Author: Zhen Peng <zhenpeng01@baidu.com>

Closes #854 from zhpengg/bugfix-worker-kills-executor and squashes the following commits:

21d380b [Zhen Peng] add some error messages
506cea6 [Zhen Peng] add some docs for killProcess()
a0b9860 [Zhen Peng] [SPARK-1901] worker should make sure executor has exited before updating executor's info