[SPARK-17451][CORE] CoarseGrainedExecutorBackend should inform driver before self-kill

## What changes were proposed in this pull request?

Jira : https://issues.apache.org/jira/browse/SPARK-17451

`CoarseGrainedExecutorBackend` in some failure cases exits the JVM. While this does not have any issue, from the driver UI there is no specific reason captured for this. In this PR, I am adding functionality to `exitExecutor` to notify driver that the executor is exiting.

## How was this patch tested?

Ran the change over a test env and took down shuffle service before the executor could register to it. In the driver logs, where the job failure reason is mentioned (ie. `Job aborted due to stage ...` it gives the correct reason:

Before:
`ExecutorLostFailure (executor ZZZZZZZZZ exited caused by one of the running tasks) Reason: Remote RPC client disassociated. Likely due to containers exceeding thresholds, or network issues. Check driver logs for WARN messages.`

After:
`ExecutorLostFailure (executor ZZZZZZZZZ exited caused by one of the running tasks) Reason: Unable to create executor due to java.util.concurrent.TimeoutException: Timeout waiting for task.`

Author: Tejas Patil <tejasp@fb.com>

Closes #15013 from tejasapatil/SPARK-17451_inform_driver.