docs: function loadContextCallback(contextPath, logger) {
      var cb;
      try {
          cb = require(contextPath);
          if ('function' !== typeof cb) {
              throw new TypeError('loadContextCallback: context callback ' +
                                  'must be function. Found: ' + cb + '. Path: ' +
                                  contextPath);
          }
      }
      catch(e) {
          logger.error('loadContextCallback: error loading ' +
                       'context file: ' + contextPath + ' ' + e.stack);
          return false;
      }
      return cb;
  }