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.
[SPARK-4057] Use -agentlib instead of -Xdebug in sbt-launch-lib.bash for debugging

In -launch-lib.bash, -Xdebug option is used for debugging. We should use -agentlib option for Java 6+.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2904 from sarutak/SPARK-4057 and squashes the following commits:

39b5320 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-4057
26b4af8 [Kousuke Saruta] Improved java option for debugging