Default executor exits with a stack trace in a few scenarios.

This happened due to MESOS-6296 accidentally making the 1.2 release. In some scenarios, the default executor commits suicide (as expected) but does it with a stack trace due to a failed invariant check.

The error scenarios happen when some task(s) in the task group terminated successfully, and if the other task(s) thereafter either were killed or terminated with a non-zero status code.

e.g., Task 1 terminated with status 0 , Task 2 was killed by the scheduler. In this scenario, the default executor would commit suicide with a stack trace.