Prevent initial ZK timeouts from killing the executor.

In addition, prevent uncaught exceptions from killing the executor.

Testing Done:
./pants src/test/python/apache/aurora/executor:executor-small
./src/test/sh/org/apache/aurora/e2e/test_end_to_end.sh

Bugs closed: AURORA-728

Reviewed at https://reviews.apache.org/r/25974/