update mysql module notice and throw as an exception

lib/dialects/mysql/connector-manager.js

@@ -13,7 +13,7 @@ module.exports = (function() {
         mysql = require('mysql')
       }
     } catch (err) {
-      console.log('You need to install mysql package manually')
+      throw new Error('Please install mysql package manually')
     }
 
     this.sequelize = sequelize