[SPARK-1886] check executor id existence when executor exit

Author: Zhen Peng <zhenpeng01@baidu.com>

Closes #827 from zhpengg/bugfix-executor-id-not-found and squashes the following commits:

cd8bb65 [Zhen Peng] bugfix: check executor id existence when executor exit

(cherry picked from commit 4e4831b8facc186cda6ef31040ccdeab48acbbb7)
Signed-off-by: Aaron Davidson <aaron@databricks.com>