as an example, the validation error has a placeholder named attribute
```
coerce: 'invalid parameter: %{attribute}'
```
when locale is en:  'invalid parameter: age' 
when locale is zh-CN: '参数不合法:  年龄'
so translate the attribute first. or it will be '参数不合法:  age'
but some other error attribute placeholer do not need to be translate, eg:
```
missing mime type for %{new_format}
```
the new_format attribute should not be translate.
so currently ,the validation error message is not generate by Grape::Exceptions::Base#compose_message.

maybe the compose_message need an option to figure out whether the attribute need to be translated, 
or use I18n.translate('xxx', default: {attr: attr}) to try to translate the attribute first, if nothing in the locale file, 
it will fallback the attribute key

not sure is it a litter overdo