Currently the launcher handle does not monitor the child spark-submit
process it launches; this means that if the child exits with an error,
the handle's state will never change, and an application will not know
that the application has failed.

This change adds code to monitor the child process, and changes the
handle state appropriately when the child process exits.

Tested with added unit tests.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #18877 from vanzin/SPARK-17742.