[SPARK-4379][Core] Change Exception to SparkException in checkpoint

It's better to change to SparkException. However, it's a breaking change since it will change the exception type.

Author: zsxwing <zsxwing@gmail.com>

Closes #3241 from zsxwing/SPARK-4379 and squashes the following commits:

409f3af [zsxwing] Change Exception to SparkException in checkpoint
[SPARK-4363][Doc] Update the Broadcast example

Author: zsxwing <zsxwing@gmail.com>

Closes #3226 from zsxwing/SPARK-4363 and squashes the following commits:

8109914 [zsxwing] Update the Broadcast example
[SPARK-4260] Httpbroadcast should set connection timeout.

Httpbroadcast sets read timeout but doesn't set connection timeout.

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

Closes #3122 from sarutak/httpbroadcast-timeout and squashes the following commits:

c7f3a56 [Kousuke Saruta] Added Connection timeout for Http Connection to HttpBroadcast.scala