Include the key name when failing on an invalid value.

Admittedly a really small tweak.

Author: Stephen Haberman <stephen@exigencecorp.com>

Closes #3514 from stephenh/include-key-name-in-npe and squashes the following commits:

937740a [Stephen Haberman] Include the key name when failing on an invalid value.

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

@@ -61,7 +61,7 @@ class SparkConf(loadDefaults: Boolean) extends Cloneable with Logging {
       throw new NullPointerException("null key")
     }
     if (value == null) {
-      throw new NullPointerException("null value")
+      throw new NullPointerException("null value for " + key)
     }
     settings(key) = value
     this