This will return a `204`, not `400`.  While we are at it we should verify a `404` for `GET /search/index/Index`.

```
20:27:42.242 [error]
================ yz_index_admin failure stack trace =====================
{{assertEqual_failed,[{module,yz_index_admin},
                      {line,239},
                      {expression,"Status"},
                      {expected,"400"},
                      {value,"204"}]},
 [{yz_index_admin,'-confirm_create_index_bad_schema/1-fun-3-',2,
                  [{file,"riak_test/yz_index_admin.erl"},{line,239}]},
  {yz_index_admin,confirm_create_index_bad_schema,1,
                  [{file,"riak_test/yz_index_admin.erl"},{line,239}]},
  {yz_index_admin,confirm,0,
                  [{file,"riak_test/yz_index_admin.erl"},{line,142}]},
  {riak_test_runner,return_to_exit,3,
                    [{file,"src/riak_test_runner.erl"},{line,159}]}]}
=========================================================================
```