[SPARK-4626] Kill a task only if the executorId is (still) registered with the scheduler

Author: roxchkplusony <roxchkplusony@gmail.com>

Closes #3483 from roxchkplusony/bugfix/4626 and squashes the following commits:

aba9184 [roxchkplusony] replace warning message per review
5e7fdea [roxchkplusony] [SPARK-4626] Kill a task only if the executorId is (still) registered with the scheduler

(cherry picked from commit 84376d31392858f7df215ddb3f05419181152e68)
Signed-off-by: Reynold Xin <rxin@databricks.com>