Get URL by key, the value must be exist, otherwise the process will exit.

@param {KeyItem} key
@param {Boolean} [endWithSeparator=false] If set ture, the key must be end with `/`.
@returns {String}
function(key,endWithSeparator) {
        var url = this.loadNecessaryString(key);
        if (!url) {
            exitProcess('empty url for ' + key, this.alarm);
            return false;
        }
        if (!url.startWith('http://') && !url.startWith('https://')) {
            exitProcess('invalid url:' + url, this.alarm);
            return false;
        }
        if (endWithSeparator && !url.endWith('/')) {
            exitProcess('the url['+url+'] must be end with /', this.alarm);
            return false;
        }
        if (!endWithSeparator && url.endWith('/')) {
            exitProcess('the url['+url+'] must not be end with /', this.alarm);
            return false;
        }
        return url;
    }