buildString(aNeededMessages) {
			const aMessages = aNeededMessages
				? aNeededMessages.map(function (i) { return this.aMessages[i]; }, this)
				: this.aMessages;
			const oMessages = Object.assign({}, aMessages[0]);
			oMessages.details = aMessages.slice(1);

			return JSON.stringify(oMessages);
		}
Builds a string of the current array of messages.

@param {array} aNeededMessages
  The indices of the needed messages; if not given all current messages are used
@returns {string}
  The message object as string