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.
I'm going to allow any other error to just break and bubble to the surface. Adding the {error, _} case gives us two options: set `ok` and continue, which would just push the problem to line 225. Or, throw a different error, which is immediately caught by this function anyway.