Add StageError

This adds the StageError Error. This Error indicates that a
Stage/Phase has hit an unrecoverable error, it cannot continue, and
requests that the entire Stage/Phase hierarchy be killed with an
ExitFailure ExitCode. StageMain is modified to catch StageError and
exit the application with the provided exit code number.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>