Fix reip loading of riak_core

Fixes basho/riak#554, reip throwing an a badmatch when attempting to load riak_core, which was already loaded by cuttlefish.
As I understand it, we can safely ignore {error, {already_loaded, riak_core}}.  However, the  [application:load/1](http://www.erlang.org/doc/apps/kernel/application.html#load-1) specifies {error, Reason }.  How should {error, _} be handled?