usecase: support multiple workers in NGFW

Improve error handling in other use cases.

Signed-off-by: Robin Shapley <Robin.Shapley@arm.com>
Signed-off-by: Paul Szczepanek <paul.szczepanek@arm.com>
Signed-off-by: Sanjyot Vaidya <Sanjyot.Vaidya@arm.com>
Signed-off-by: Luca Vizzarro <luca.vizzarro@arm.com>