Display validation errors on login form

Closes #1464.

I've added small changes that display error message on the login form.
This is the original state of the login form:

![login_form](https://cloud.githubusercontent.com/assets/7045848/4149745/c5102d1a-3437-11e4-8d80-7426697d093f.png)

If bad credentials are provided, corresponding error message is displayed:

![login_bad_credentials](https://cloud.githubusercontent.com/assets/7045848/4149758/dcfcf534-3437-11e4-9bbe-de927f9c638c.png)

If 'Username' or 'Password' fields are empty HTML5 validation applies:

![login_empty_user](https://cloud.githubusercontent.com/assets/7045848/4149766/f62e56d8-3437-11e4-8839-4e198e1d208e.png)

![login_empty_pass](https://cloud.githubusercontent.com/assets/7045848/4149773/fa91e816-3437-11e4-8a50-cc01652b4e7d.png)

If the browser doesn't support HTML5 form validation, then this would be expected to display when the user submits an empty form:
![login_form_empty_fields](https://cloud.githubusercontent.com/assets/7045848/4149783/0fa8864c-3438-11e4-9a53-2eb7c5c2315e.png)

Let's use `text-error` rather than `text-warning`