cli.JobExecutor.WaitOrShow: always return status

Currently, for the 'wait' case, we return a list of tuples (status,
result), in the order of submitted jobs, but we don't return anything
for the no-wait case.

This patch changes the no-wait case to return a list of tuples (status,
result), where result can be either a job ID or an error message.
Processing in clients can then ignore whether we did wait or not, and
test the overall or individual status.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>