[SPARK-5126][Core] Verify Spark urls before creating Actors so that invalid...

[SPARK-5126][Core] Verify Spark urls before creating Actors so that invalid urls can crash the process.

Because `actorSelection` will return `deadLetters` for an invalid path,  Worker keeps quiet for an invalid master url. It's better to log an error so that people can find such problem quickly.

This PR will check the url before sending to `actorSelection`, throw and log a SparkException for an invalid url.

Author: zsxwing <zsxwing@gmail.com>

Closes #3927 from zsxwing/SPARK-5126 and squashes the following commits:

9d429ee [zsxwing] Create a utility method in Utils to parse Spark url; verify urls before creating Actors so that invalid urls can crash the process.
8286e51 [zsxwing] Check the url before sending to Akka and log the error if the url is invalid