make built-in validators return an instance of Error vs string

lib/dao-validator.js

@@ -402,11 +402,6 @@ DaoValidator.prototype._pushError = function(isBuiltin, errorKey, rawError) {
     this.errors[errorKey] = [];
   }
 
-  if (isBuiltin) {
-    this.errors[errorKey].push(rawError);
-    return;
-  }
-
   var error = new sequelizeError.ValidationError()
 
   error[DaoValidator.RAW_KEY_NAME] = rawError