Don't rollback twice on connection error

lib/sequelize.js

@@ -1239,6 +1239,9 @@ Sequelize.prototype.transaction = function(options, autoCallback) {
         // If an error occured while commiting a transaction do not try to rollback the transaction
         if (transaction.finished === 'commit') {
           reject(err);
+        } else if (transaction.finished === 'rollback'){
+          // Already in rollback state. Reject with the original error.
+          reject(err);
         } else {
           transaction.rollback().finally(function () {
             reject(err);