Improve login page styling

- Use the narrow column for the whole page, not just the form
- Update styles accordingly
- Simplify error output and include it all inside the styled error element

![screen shot 2014-10-21 at 2 33 09 pm](https://cloud.githubusercontent.com/assets/2289/4727223/4404e046-596a-11e4-950e-7b5d9102fc60.png)