Give Databricks-specific messages when new SparkContext or SQLContext is created

## What changes were proposed in this pull request?
This commit hard-codes allowMultipleContexts to false and change the error message to a DBC-specific one. This is originally authored by yhuai.

Author: Yin Huai <yhuai@databricks.com>

Closes apache#43 from rxin/rxin-dbc-sparkcontext-msg.

Author: yhuai

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -2215,9 +2215,11 @@ object SparkContext extends Logging {
       allowMultipleContexts: Boolean): Unit = {
     SPARK_CONTEXT_CONSTRUCTOR_LOCK.synchronized {
       Option(activeContext.get()).filter(_ ne sc).foreach { ctx =>
-          val errMsg = "Only one SparkContext may be running in this JVM (see SPARK-2243)." +
-            " To ignore this error, set spark.driver.allowMultipleContexts = true. " +
-            s"The currently running SparkContext was created at:\n${ctx.creationSite.longForm}"
+          val ctx = activeContext.get()
+          val errMsg = "In Databricks, developers should utilize the shared SparkContext " +
+            "instead of creating one using the constructor. In Scala and Python notebooks, " +
+            "the shared context can be accessed as sc. When running a job, " +
+            "you can access the shared context by calling SparkContext.getOrCreate()."
           val exception = new SparkException(errMsg)
           if (allowMultipleContexts) {
             logWarning("Multiple running SparkContexts detected in the same JVM!", exception)
@@ -2226,16 +2228,18 @@ object SparkContext extends Logging {
           }
         }
 
-      contextBeingConstructed.filter(_ ne sc).foreach { otherContext =>
-        // Since otherContext might point to a partially-constructed context, guard against
-        // its creationSite field being null:
-        val otherContextCreationSite =
-          Option(otherContext.creationSite).map(_.longForm).getOrElse("unknown location")
-        val warnMsg = "Another SparkContext is being constructed (or threw an exception in its" +
-          " constructor).  This may indicate an error, since only one SparkContext may be" +
-          " running in this JVM (see SPARK-2243)." +
-          s" The other SparkContext was created at:\n$otherContextCreationSite"
-        logWarning(warnMsg)
+      contextBeingConstructed.foreach { otherContext =>
+        if (otherContext ne sc) {  // checks for reference equality
+          // Since otherContext might point to a partially-constructed context, guard against
+          // its creationSite field being null:
+          val otherContextCreationSite =
+            Option(otherContext.creationSite).map(_.longForm).getOrElse("unknown location")
+          val warnMsg = "Another SparkContext is being constructed (or threw an exception in its" +
+            " constructor).  This may indicate an error, since only one SparkContext may be" +
+            " running in this JVM (see SPARK-2243)." +
+            s" The other SparkContext was created at:\n$otherContextCreationSite"
+          logWarning(warnMsg)
+        }
       }
     }
   }