[SPARK-20940][CORE] Replace IllegalAccessError with IllegalStateException

## 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.

core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala

@@ -68,7 +68,7 @@ abstract class AccumulatorV2[IN, OUT] extends Serializable {
 
   private def assertMetadataNotNull(): Unit = {
     if (metadata == null) {
-      throw new IllegalAccessError("The metadata of this accumulator has not been assigned yet.")
+      throw new IllegalStateException("The metadata of this accumulator has not been assigned yet.")
     }
   }
 
@@ -265,7 +265,7 @@ private[spark] object AccumulatorContext {
       // Since we are storing weak references, we must check whether the underlying data is valid.
       val acc = ref.get
       if (acc eq null) {
-        throw new IllegalAccessError(s"Attempted to access garbage collected accumulator $id")
+        throw new IllegalStateException(s"Attempted to access garbage collected accumulator $id")
       }
       acc
     }

core/src/test/scala/org/apache/spark/AccumulatorSuite.scala

@@ -210,7 +210,7 @@ class AccumulatorSuite extends SparkFunSuite with Matchers with LocalSparkContex
     assert(ref.get.isEmpty)
 
     // Getting a garbage collected accum should throw error
-    intercept[IllegalAccessError] {
+    intercept[IllegalStateException] {
       AccumulatorContext.get(accId)
     }