## What changes were proposed in this pull request?

`IllegalAccessError` is a fatal error (a subclass of LinkageError) and its meaning is `Thrown if an application attempts to access or modify a field, or to call a method that it does not have access to`. Throwing a fatal error for AccumulatorV2 is not necessary and is pretty bad because it usually will just kill executors or SparkContext ([SPARK-20666](https://issues.apache.org/jira/browse/SPARK-20666) is an example of killing SparkContext due to `IllegalAccessError`). I think the correct type of exception in AccumulatorV2 should be `IllegalStateException`.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18168 from zsxwing/SPARK-20940.