Fix NameError for `412Error` on 412 without body

```
>> Aws::S3.new.head_object(if_unmodified_since: '2014-06-09 12:44:21 +0900',
bucket: '...', key: '...') NameError: wrong constant name 412Error
```

by the way, current implementation of `ErrorHandler#.error_code_for_empty_response` makes invalid string for constant name (ex: `412Error`)when pre-defined error constant not found, is this expected?