use 401 instead of 500/400 for empty logins

Loggin in without a user or password returns:

```
http://localhost:4200/api/v1/auths/login 500 (Internal Server Error)
```

Wrong user or password returns:

```
http://localhost:4200/api/v1/auths/login 400 (Bad Request)
```

It should have been a [401 Unauthorized](http://httpstatusdogs.com/401-unauthorized)
