[SPARK-17802] Improved caller context logging.

## What changes were proposed in this pull request?

[SPARK-16757](https://issues.apache.org/jira/browse/SPARK-16757) sets the hadoop `CallerContext` when calling hadoop/hdfs apis to make spark applications more diagnosable in hadoop/hdfs logs. However, the `org.apache.hadoop.ipc.CallerContext` class is only added since [hadoop 2.8](https://issues.apache.org/jira/browse/HDFS-9184), which is not officially releaed yet. So each time `utils.CallerContext.setCurrentContext()` is called (e.g [when a task is created](https://github.com/apache/spark/blob/b678e46/core/src/main/scala/org/apache/spark/scheduler/Task.scala#L95-L96)), a "java.lang.ClassNotFoundException: org.apache.hadoop.ipc.CallerContext"
error is logged, which pollutes the spark logs when there are lots of tasks.

This patch improves this behaviour by only logging the `ClassNotFoundException` once.

## How was this patch tested?

Existing tests.

Author: Shuai Lin <linshuai2012@gmail.com>

Closes #15377 from lins05/spark-17802-improve-callercontext-logging.