fix(connection): emit error if no promise handler

Fix #4135

lib/connection.js

@@ -279,19 +279,20 @@ Connection.prototype.open = function(host, database, port, options, callback) {
 
   var _this = this;
   var Promise = PromiseProvider.get();
-  return new Promise.ES6(function(resolve, reject) {
-    _this._open(!!callback, function(error) {
+  var promise = new Promise.ES6(function(resolve, reject) {
+    _this._open(true, function(error) {
       callback && callback(error);
       if (error) {
         reject(error);
-        if (!callback && _this.listeners('error').length) {
+        if (!callback && !promise.$hasHandler) {
           _this.emit('error', error);
         }
         return;
       }
       resolve();
     });
   });
+  return promise;
 };
 
 /**
@@ -416,19 +417,20 @@ Connection.prototype.openSet = function(uris, database, options, callback) {
 
   var _this = this;
   var Promise = PromiseProvider.get();
-  return new Promise.ES6(function(resolve, reject) {
-    _this._open(!!callback, function(error) {
+  var promise = new Promise.ES6(function(resolve, reject) {
+    _this._open(true, function(error) {
       callback && callback(error);
       if (error) {
         reject(error);
-        if (!callback && _this.listeners('error').length) {
+        if (!callback && !promise.$hasHandler) {
           _this.emit('error', error);
         }
         return;
       }
       resolve();
     });
   });
+  return promise;
 };
 
 /**
@@ -475,7 +477,7 @@ Connection.prototype._open = function(emit, callback) {
           callback(err);
         }
       } else {
-        _this.error(err, callback);
+        _this.error(err, emit && callback);
       }
       return;
     }

lib/index.js

@@ -766,6 +766,7 @@ MongooseThenable.prototype.then = function(onFulfilled, onRejected) {
         'has been called'));
     }).then(onFulfilled, onRejected);
   }
+  this.$opPromise.$hasHandler = true;
   return this.$opPromise.then(onFulfilled, onRejected);
 };