hbal: return meaningful exit code for job failures

Currently, LUXI job failures only display a warning message, while
still returning a success exit code. We change hbal to return
true/false from within execJobSet/runJobSet, and add a wrapper for
simpler code.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Adeodato Simo <dato@google.com>