Return response from vnode_master:sync_command

Pair-programmed with @reiddraper

Previously this function always returned 'ok', but its caller expects an
error to sometimes be returned. The intention appears to be to return an
error if the sync_command fails.

Fixes dialyer error:
riak_kv_util.erl:294: The pattern 'error' can never match the type 'ok'
| 'undefined'