[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.
@srowen It looks like this same `exception` URL param is used in other exception-handling code in this same file (the first instance was added by @andrewor14 in 6afca2d1079bac6309a595b8e0ffc74ae93fa662).