FIX UploadField server validation error display

While working on a solution for a StackOverflow question about custom Upload validation (http://stackoverflow.com/questions/18242663/silverstripe-custom-validator-on-uploadfield/18249650) came across a bug where UploadField would not display the validation error message but Forbidden instead.

Removed the `$response->setStatusCode(403)` in the upload response so error messages are now displayed.

Updated the download template and the CSS so the error messages look a bit better.
Before:
![image](https://f.cloud.github.com/assets/978040/996360/804330e0-09ce-11e3-9b9a-cb719b6b62be.png)
![image](https://f.cloud.github.com/assets/978040/996362/88957f46-09ce-11e3-9724-38561305dc0b.png)
![image](https://f.cloud.github.com/assets/978040/996365/8defaf20-09ce-11e3-84b9-b484bc9e295a.png)

After:
![image](https://f.cloud.github.com/assets/978040/996368/967f794a-09ce-11e3-9121-3813bd1247b1.png)
![image](https://f.cloud.github.com/assets/978040/996369/9c38e948-09ce-11e3-88fa-d7d77028b077.png)
![image](https://f.cloud.github.com/assets/978040/996373/a3e6d088-09ce-11e3-8d11-93fd514c5646.png)

This is open for review and discussion and more tests. And all commits can be squashed before/if merging.