[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