code: function getChildNode(node, tagName, errorMsgIfAbsent) {
      if (node && node.childNodes) {
          for (var i = 0, cn = node.childNodes, len = cn.length; i < len; i++) {
              if (cn[i].tagName === tagName) {
                  return cn[i];
              }
          }
      }
      if (errorMsgIfAbsent) {
          throw new KdbxError(Consts.ErrorCodes.FileCorrupt, errorMsgIfAbsent);
      } else {
          return null;
      }
  }
docs: Gets first child node from xml
  @param {Node} node - parent node for search
  @param {string} tagName - child node tag name
  @param {string} [errorMsgIfAbsent] - if set, error will be thrown if node is absent
  @returns {Node} - first found node, or null, if there's no such node