Ignore a task update status if the executor doesn't exist anymore.
Merge pull request #143 from rxin/scheduler-hang

Ignore a task update status if the executor doesn't exist anymore.

Otherwise if the scheduler receives a task update message when the executor's been removed, the scheduler would hang.

It is pretty hard to add unit tests for these right now because it is hard to mock the cluster scheduler. We should do that once @kayousterhout finishes merging the local scheduler and the cluster scheduler.