Report errors in tasks to the driver via a Mesos status update

When a task throws an exception, the Spark executor previously just
logged it to a local file on the slave and exited. This commit causes
Spark to also report the exception back to the driver using a Mesos
status update, so the user doesn't have to look through a log file on
the slave.

Here's what the reporting currently looks like:

    # ./run spark.examples.ExceptionHandlingTest master@203.0.113.1:5050
    [...]
    11/10/26 21:04:13 INFO spark.SimpleJob: Lost TID 1 (task 0:1)
    11/10/26 21:04:13 INFO spark.SimpleJob: Loss was due to java.lang.Exception: Testing exception handling
    [...]
    11/10/26 21:04:16 INFO spark.SparkContext: Job finished in 5.988547328 s