susbsys: settings: fix coverity issues

API settings_subsys_init call was changed so that it returns
error (so returns int instead of void).
Prototype of storage helper function export_func for
settings_handler::h_export was changed so that it returns error
(so returns int instead of void).
Fixed few other error handling issues by ignoring return
values.

Tests were aligned to above patches.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>