我试图在spark中运行简单的字数计算作业，但在运行作业时遇到异常。

For more detailed output, check application tracking page:http://quickstart.cloudera:8088/proxy/application_1446699275562_0006/Then, click on links to logs of each attempt.
Diagnostics: Exception from container-launch.
Container id: container_1446699275562_0006_02_000001
Exit code: 15
Stack trace: ExitCodeException exitCode=15:
        at org.apache.hadoop.util.Shell.runCommand(Shell.java:538)
        at org.apache.hadoop.util.Shell.run(Shell.java:455)
        at org.apache.hadoop.util.Shell$ShellCommandExecutor.execute(Shell.java:715)
        at org.apache.hadoop.yarn.server.nodemanager.DefaultContainerExecutor.launchContainer(DefaultContainerExecutor.java:211)
        at org.apache.hadoop.yarn.server.nodemanager.containermanager.launcher.ContainerLaunch.call(ContainerLaunch.java:302)
        at org.apache.hadoop.yarn.server.nodemanager.containermanager.launcher.ContainerLaunch.call(ContainerLaunch.java:82)
        at java.util.concurrent.FutureTask.run(FutureTask.java:262)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)

Container exited with a non-zero exit code 15
Failing this attempt. Failing the application.
         ApplicationMaster host: N/A
         ApplicationMaster RPC port: -1
         queue: root.cloudera
         start time: 1446910483956
         final status: FAILED
         tracking URL: http://quickstart.cloudera:8088/cluster/app/application_1446699275562_0006
         user: cloudera
Exception in thread "main" org.apache.spark.SparkException: Application finished with failed status
        at org.apache.spark.deploy.yarn.Client.run(Client.scala:626)
        at org.apache.spark.deploy.yarn.Client$.main(Client.scala:651)
        at org.apache.spark.deploy.yarn.Client.main(Client.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:569)
        at org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:166)
        at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:189)
        at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:110)
        at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

我检查了以下命令的日志

yarn logs -applicationId application_1446699275562_0006

这是日志

15/11/07 07:35:09 ERROR yarn.ApplicationMaster: User class threw exception: Output directory hdfs://quickstart.cloudera:8020/user/cloudera/WordCountOutput already exists
org.apache.hadoop.mapred.FileAlreadyExistsException: Output directory hdfs://quickstart.cloudera:8020/user/cloudera/WordCountOutput already exists
        at org.apache.hadoop.mapred.FileOutputFormat.checkOutputSpecs(FileOutputFormat.java:132)
        at org.apache.spark.rdd.PairRDDFunctions.saveAsHadoopDataset(PairRDDFunctions.scala:1053)
        at org.apache.spark.rdd.PairRDDFunctions.saveAsHadoopFile(PairRDDFunctions.scala:954)
        at org.apache.spark.rdd.PairRDDFunctions.saveAsHadoopFile(PairRDDFunctions.scala:863)
        at org.apache.spark.rdd.RDD.saveAsTextFile(RDD.scala:1290)
        at org.com.td.sparkdemo.spark.WordCount$.main(WordCount.scala:23)
        at org.com.td.sparkdemo.spark.WordCount.main(WordCount.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.apache.spark.deploy.yarn.ApplicationMaster$$anon$2.run(ApplicationMaster.scala:480)
15/11/07 07:35:09 INFO yarn.ApplicationMaster: Final app status: FAILED, exitCode: 15, (reason: User class threw exception: Output directory hdfs://quickstart.cloudera:8020/user/cloudera/WordCountOutput already exists)
15/11/07 07:35:14 ERROR yarn.ApplicationMaster: SparkContext did not initialize after waiting for 100000 ms. Please check earlier log output for errors. Failing the application.

异常清楚地表明wordcountoutput目录已经存在，但在运行作业之前，我确保该目录不存在。
为什么我得到这个错误，即使目录不存在之前运行我的工作？