Merge pull request #320 from kayousterhout/erroneous_failed_msg

Remove erroneous FAILED state for killed tasks.

Currently, when tasks are killed, the Executor first sends a
status update for the task with a "KILLED" state, and then
sends a second status update with a "FAILED" state saying that
the task failed due to an exception. The second FAILED state is
misleading/unncessary, and occurs due to a NonLocalReturnControl
Exception that gets thrown due to the way we kill tasks. This
commit eliminates that problem.

I'm not at all sure that this is the best way to fix this problem,
so alternate suggestions welcome. @rxin guessing you're the right
person to look at this.

(cherry picked from commit 0475ca8f81b6b8f21fdb841922cd9ab51cfc8cc3)
Signed-off-by: Reynold Xin <rxin@apache.org>