Fix race condition (COMPLETED/BOT_DIED) in task_runner.

Per b/69462084 and a TODO comment in this file, a race condition
exists when an external process polls task status that fails with
BOT_DIED (there's a short window in which the task will be marked
as COMPLETED before being updated to BOT_DIED). This change sends
the task-update without the exit code (to prevent the COMPLETED
status from being set), and then sends a task_error message. The
default must_signal_internal_failure is also changed to provide a
default error in case run_isolated fails.

TESTED=Ran unit tests in task_runner_test and they pass.

Any other tests?

Change-Id: I1db3e15ccbdac3da9181b273681180211b07841c
Reviewed-on: https://chromium-review.googlesource.com/834397
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Mirrored-From: https://chromium.googlesource.com/infra/luci/luci-py
Cr-Mirrored-Commit: 0b027452e658080df1f174c403946914443d2aa6