fix(connection): throw helpful error when using legacy `mongoose.conn…

…ect()` syntax

Fix #6756

lib/connection.js

@@ -414,6 +414,13 @@ Connection.prototype.openUri = function(uri, options, callback) {
     options = null;
   }
 
+  if (['string', 'number'].indexOf(typeof options) !== -1) {
+    throw new MongooseError('Mongoose 5.x no longer supports ' +
+      '`mongoose.connect(host, dbname, port)` or ' +
+      '`mongoose.createConnection(host, dbname, port)`. See ' +
+      'http://mongoosejs.com/docs/connections.html for supported connection syntax');
+  }
+
   const Promise = PromiseProvider.get();
   const _this = this;