code: function(key,onlyCheckDirectory) {
          var filePath = this.loadVar(key);
          if (!filePath) {
              exitProcess('empty file path for ' + key, this.alarm);
              return false;
          }
          if (!onlyCheckDirectory) {
              if (!fs.existsSync(filePath)) {
                  exitProcess('the value of ' +key+' is a necessary file ,but not exists in '+ filePath, this.alarm);
                  return false;
              }
          } else {
              var dirname = path.dirname(filePath);
              if (!fs.lstatSync(dirname).isDirectory()) {
                  exitProcess('the path '+dirname + ' must exist and be a directory', this.alarm);
                  return false;
              }
          }
  
          return filePath;
      }
docs: Get file path by key, the value must be exist, otherwise the process will exit.
  
  @param {KeyItem} key
  @param {Boolean} [onlyCheckDirectory=false] If set true, only check whether the directory that the file saved is exist.
  @returns {String}