Fix transaction not rolling back if no thenable was provided

changelog.md

 # next
-- [BUG] Fix regression introduced in 2.1.2: updatedAt not set anymore
+- [BUG] Fix regression introduced in 2.1.2: updatedAt not set anymore [3667](https://github.com/sequelize/sequelize/pull/3667)
+- [BUG] Fix managed transactions not rolling back if no thenable was provided in the transaction block [3667](https://github.com/sequelize/sequelize/pull/3667)
 
 # 2.1.2
 - [BUG] `Model.create()/update()` no longer attempts to save undefined fields.

lib/sequelize.js

@@ -1140,7 +1140,7 @@ module.exports = (function() {
           }
 
           var result = autoCallback(transaction);
-          if (!result || !result.then) return reject(new Error('You need to return a promise chain/thenable to the sequelize.transaction() callback'));
+          if (!result || !result.then) throw new Error('You need to return a promise chain/thenable to the sequelize.transaction() callback');
 
           return result.then(function (result) {
             return transaction.commit().then(function () {
@@ -1148,9 +1148,7 @@ module.exports = (function() {
             });
           });
         }).catch(function(err) {
-          transaction.rollback().then(function () {
-            reject(err);
-          }, function () {
+          transaction.rollback().finally(function () {
             reject(err);
           });
         });