Correct error message to show exact module name

lib/dialects/postgres/connection-manager.js

@@ -14,7 +14,7 @@ ConnectionManager = function(dialect, sequelize) {
   try {
     this.lib = sequelize.config.native ? require(sequelize.config.dialectModulePath || 'pg').native : require(sequelize.config.dialectModulePath || 'pg');
   } catch (err) {
-    throw new Error('Please install postgres package manually');
+    throw new Error('Please install \'' + (sequelize.config.dialectModulePath || 'pg') + '\' module manually');
   }
 };