[SPARK-11709] include creation site info in SparkContext.assertNotStopped error message

This helps debug issues caused by multiple SparkContext instances. JoshRosen andrewor14

~~~
scala> sc.stop()

scala> sc.parallelize(0 until 10)
java.lang.IllegalStateException: Cannot call methods on a stopped SparkContext.
This stopped SparkContext was created at:

org.apache.spark.SparkContext.<init>(SparkContext.scala:82)
org.apache.spark.repl.SparkILoop.createSparkContext(SparkILoop.scala:1017)
$iwC$$iwC.<init>(<console>:9)
$iwC.<init>(<console>:18)
<init>(<console>:20)
.<init>(<console>:24)
.<clinit>(<console>)
.<init>(<console>:7)
.<clinit>(<console>)
$print(<console>)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:606)
org.apache.spark.repl.SparkIMain$ReadEvalPrint.call(SparkIMain.scala:1065)
org.apache.spark.repl.SparkIMain$Request.loadAndRun(SparkIMain.scala:1340)
org.apache.spark.repl.SparkIMain.loadAndRunReq$1(SparkIMain.scala:840)
org.apache.spark.repl.SparkIMain.interpret(SparkIMain.scala:871)
org.apache.spark.repl.SparkIMain.interpret(SparkIMain.scala:819)
org.apache.spark.repl.SparkILoop.reallyInterpret$1(SparkILoop.scala:857)

The active context was created at:

(No active SparkContext.)
~~~

Author: Xiangrui Meng <meng@databricks.com>

Closes #9675 from mengxr/SPARK-11709.