Return master ip script output in case of failure

Currently only the exit code is returned, which is somewhat pointless:
for failures, we already know it's ≠ 0, so we don't have enough
information (reported on the mailing list). By logging the output as
well (if any), the user will have more information available for
debugging.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>