Added Error Descriptions

I often find the **description** field on **HTTPException**s in the [Werkzeug](https://github.com/mitsuhiko/werkzeug) library very handy. These are simple error explanations, not phrased for the programmer as docstrings typically are, but rather for the end user. For that reason, one can just drop them into error pages, and end users get detailed explanations of their errors.

I found myself making a table of descriptions for the **Error** classes in [PyBrowserID](https://github.com/mozilla/PyBrowserID) for a project, but then I realized this sort of thing should be a pull request.

This also resolves some of the concerns expressed in #22.