Add proper error message for invalid credentials

- This adds a nicer error message if the user entered an invalid email and/or password
- Displays a different error message if the issue was network-related
- Displays a different error message if the issue was a bad server response

Here's an example (first 2 cases)
![ios simulator screen shot 03 apr 2014 19 49 32](https://cloud.githubusercontent.com/assets/124243/2607081/c4469598-bb58-11e3-8e3d-bc8c86724fdf.png)
![ios simulator screen shot 03 apr 2014 19 50 03](https://cloud.githubusercontent.com/assets/124243/2607084/c767a01e-bb58-11e3-9144-e78e90e270f1.png)