Bug: Fix login algorithm

[victorbrender]

Steps to reproduce:

• when trying to login with a wrong username the application raises an error TypeError: Cannot read property 'password' of undefined on line https://github.com/Learn-by-doing/bitstarter/blob/master/controllers/login.js#L61

Expected result:

• application should gracefully handle a wrong username and send an error message to the view

[thymdi]

Issue not reproduced:
Wrong user name is being handled by the application - error message send to the view.