Abort when --post-flight command fails

[aureliojargas]

Previously, any error when running the --post-flight command was
ignored. The rationale was that all the "real" tests were already
executed, being the post-flight a mere non-fatal cleanup action.

Now any failure in that step is fatal.
Just like it already is with --pre-flight.

The user specifically asked for a cleanup task to be run, and should be
notified on any error during its execution. If this is an optional task
for the user, he/she can just add a || true at the end so that it
always succeeds, even in case of errors.