code: function _buildErrorCode (cartoError, key) {
    var fragments = [];
    fragments.push(cartoError && cartoError.origin);
    fragments.push(cartoError && cartoError.type);
    fragments.push(key);
    fragments = _.compact(fragments);
  
    return fragments.join(':');
  }
docs: Generate an unique string that represents a cartoError
  @param {cartoError} cartoError
  @param {string} key