[SPARK-3193]output errer info when Process exit code is not zero in test suite

https://issues.apache.org/jira/browse/SPARK-3193
I noticed that sometimes pr tests failed due to the Process exitcode != 0,refer to
https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/18688/consoleFull
https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/19118/consoleFull

[info] SparkSubmitSuite:
[info] - prints usage on empty input
[info] - prints usage with only --help
[info] - prints error with unrecognized options
[info] - handle binary specified but not class
[info] - handles arguments with --key=val
[info] - handles arguments to user program
[info] - handles arguments to user program with name collision
[info] - handles YARN cluster mode
[info] - handles YARN client mode
[info] - handles standalone cluster mode
[info] - handles standalone client mode
[info] - handles mesos client mode
[info] - handles confs with flag equivalents
[info] - launch simple application with spark-submit *** FAILED ***
[info]   org.apache.spark.SparkException: Process List(./bin/spark-submit, --class, org.apache.spark.deploy.SimpleApplicationTest, --name, testApp, --master, local, file:/tmp/1408854098404-0/testJar-1408854098404.jar) exited with code 1
[info]   at org.apache.spark.util.Utils$.executeAndGetOutput(Utils.scala:872)
[info]   at org.apache.spark.deploy.SparkSubmitSuite.runSparkSubmit(SparkSubmitSuite.scala:311)
[info]   at org.apache.spark.deploy.SparkSubmitSuite$$anonfun$14.apply$mcV$sp(SparkSubmitSuite.scala:291)
[info]   at org.apache.spark.deploy.SparkSubmitSuite$$anonfun$14.apply(SparkSubmitSuite.scala:284)
[info]   at org.apacSpark assembly has been built with Hive, including Datanucleus jars on classpath

this PR output the process error info when failed, it can be helpful for diagnosis.
You actually don't need `toString` here (not a huge deal)