pass errors to the transaction callback if two arguments are expected

lib/sequelize.js

@@ -418,12 +418,19 @@ module.exports = (function() {
   Sequelize.prototype.transaction = function(_options, _callback) {
     var options     = (typeof _options === 'function') ? {} : _options
       , callback    = (typeof _options === 'function') ? _options : _callback
+      , wantsError  = (callback.length === 2)
       , transaction = new Transaction(this, options)
       , self        = this
 
     Utils.tick(function() {
+      if (wantsError) {
+        transaction.error(function(err) {
+          callback(err, transaction)
+        })
+      }
+
       transaction.prepareEnvironment(function() {
-        callback(transaction)
+        wantsError ? callback(null, transaction) : callback(transaction)
       })
     })