Correct return type information on riak_core_bucket:set_bucket/2

In basho/riak_api#62, the dialyzer check fails because it believes the
spec on riak_core_bucket:set_bucket/2, meaning that `{error, no_type}`
would completely cover any returns matching `{error, _}`. However,
validation errors may also be returned from `set_bucket/2`, as a list
of atom pairs that resulted from the call to
riak_core_bucket_props:validate/4.