Escape error messages on the retry page

On the retry view, the error message isn't being escaped. This isn't really a problem for most error messages, but I was debugging a worker with `fail obj.inspect`, which leads to the browser trying to interpret the `<class-name>` bit.

Before:
![before](https://f.cloud.github.com/assets/48903/1411667/c837e040-3dbc-11e3-9f32-5f8881c29931.png)

After:
![after](https://f.cloud.github.com/assets/48903/1411668/ca3a71f0-3dbc-11e3-8346-ac072ce37072.png)