[SPARK-4989][CORE] avoid wrong eventlog conf cause cluster down in standalone mode

when enabling eventlog in standalone mode, if give the wrong configuration, the standalone cluster will down (cause master restart, lose connection with workers).
How to reproduce: just give an invalid value to "spark.eventLog.dir", for example: spark.eventLog.dir=hdfs://tmp/logdir1, hdfs://tmp/logdir2. This will throw illegalArgumentException, which will cause the Master restart. And the whole cluster is not available.
It looks like `eventLogFile` is only read from inside the `try` block on the following line, so why not move it inside and make it into a `val` instead?