gnt-instance: fix GenericManyOps

Currently, GenericManyOps ignores the actual success or failure results
from the invididual jobs. We change this to return '0' (i.e. success)
only when all jobs failed, as many times we have just one job.

Together with the JobExecutor change, this will report failures
correctly when used with a drained queue and submit only, or when used
normally and the opcode actually fails.

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