3.0 remove exception.rst

The `exception` page is totally empty and refers to the `error/exception` page.
![image](https://cloud.githubusercontent.com/assets/4977112/4023727/ca0f9cd6-2ba1-11e4-8303-6a7e3ca93706.png)

I propose to remove it, and I updated the toc and links.

We could also rename the `error.rst` to `error-exceptions.rst` if it does make sense