Get directory path 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 `/`(Linux) or `\`(Windows).
@returns {String}
function(key,endWithSeparator) {
        var filepath = this.loadNecessaryFile(key);
        if (!filepath) {
            exitProcess('empty directory for ' + key, this.alarm);
            return false;
        }
        if (!fs.lstatSync(filepath).isDirectory()) {
            exitProcess('the path '+filepath + ' must be a directory', this.alarm);
            return false;
        }
        if (endWithSeparator && !filepath.endWith(path.sep)) {
            exitProcess('the path '+filepath + ' must be end with a separator', this.alarm);
            return false;
        }
        return filepath;
    }