i18n_raise_exception_handler.rb

module I18n
  class JustRaiseExceptionHandler < ExceptionHandler
    def call(exception, locale, key, options)
      if exception.is_a?(MissingTranslation)
        raise exception.to_exception
      else
        super
      end
    end
  end
end

I18n.exception_handler = I18n::JustRaiseExceptionHandler.new