Fix a possible crash if operation is cancelled file adding dependent file operations
Fix a possible crash if operation is cancelled file adding dependent file operations

If the operation is cancelled, then `queueSubtaskForDescriptor` returns nil. Then it would try to add nil to an array, which was causing a crash.

Also added a few more checks for cancelled state.

mindsnacks/Zinc-ObjC,mindsnacks/Zinc-ObjC,mindsnacks/Zinc-ObjC