Proper error handling

[xldenis]

At the moment, we have a rather basic error reporting mechanism, as a result most of Creusot just panics upon an error.

Obviously this sucks: we don't produce helpful error messages. The issue is that reporting an error requires having a Session on hand (wrapped by TranslationCtx). I propose we introduce a standard Error type which allows us to build up Result<_, Error> results and emit them at specific points.