[WIP][Tests] Log failed command's stderr if an exception is thrown

The `SparkSubmitSuite` has been failing intermittently since #1777 went in. However, it fails with the unhelpful message:
```
org.apache.spark.SparkException: Process List(./bin/spark-submit, --class, org.apache.spark.deploy.SimpleApplicationTest, --name, testApp, --master, local, file:/tmp/1407359907812-0/testJar-1407359907812.jar) exited with code 1
```

Adding the failed command's stderr to the exception message will make it easier to debug these test failures.