Correct return type information on riak_core_bucket:set_bucket/2

In basho/riak_api#62, the [dialyzer check fails](http://buildbot.bos1/builders/test-riak_api/builds/13/steps/shell_5/logs/stdio) 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.