[SPARK-18523][PYSPARK] Make SparkContext.stop more reliable

## What changes were proposed in this pull request?

This PR fixes SparkContext broken state in which it may fall if spark driver get crashed or killed by OOM.

## How was this patch tested?

1. Start SparkContext;
2. Find Spark driver process and `kill -9` it;
3. Call `sc.stop()`;
4. Create new SparkContext after that;

Without this patch you will crash on step 3 and won't be able to do step 4 without manual reset private attibutes or IPython notebook / shell restart.

Author: Alexander Shorin <kxepal@apache.org>

Closes #15961 from kxepal/18523-make-spark-context-stop-more-reliable.

(cherry picked from commit 71352c94)
Signed-off-by: Reynold Xin <rxin@databricks.com>