avoid raising error when media is not found (#2059)
avoid raising error when media is not found (#2059)

to be consistent with other storages

SDESK-5653
petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core