def failure_message
              message = "expected '#{subject_as_string}' to have a translated error message for '#{@error}'\n"
              message << "but none of the following keys was found:\n"
              message << "#{translation_keys.map { |l| "  - #{l}" }.join("\n")}\n"
              message << "for the locales: #{locales_as_string(@failed_locales)}"