Switch to using spawn to properly treat errors during sync_state

Changes f1b9ac5a and 9f6ff7e3 were aimed at improving the robustness
of the sync_state method. However both missed switching from spawn_n
to spawn to properly deal with raised exceptions.

Change-Id: Ieda4a5ecca63de67d8a9757962e7e3383fd9c9ea
Related-bug: #1257514
Merge "Switch to using spawn to properly treat errors during sync_state"