docs: function validateFieldType(fieldsCollection){
          var err;
  
          // ensure fieldsCollection is an array.
          if(!_.isArray(fieldsCollection)) {
              err = new Error('content type fields collection must be an array of field objects.');
          }
  
          if(!err) {
              _.each(fieldsCollection, function(field){
                  err = isValidField(field);
              });
          }
  
          return _.isUndefined(err);
      }