code: 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;
      }
docs: 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}