Passthrough service functions: warn unused result

Prevent these from cropping up again:
b/34634573
b/34641942
b/34640335
b/34643521
b/36424561

Note, also made defaultServicePassthroughImplementation return status_t
instead of int.

Test: pass
Change-Id: I930a5f7f82390a6869e0ab99c8536bf73dad7d0c
1 file changed