Remove dependency on lxml to render error messages

We have FormEncode-Jinja2 which includes error message display in
its formfill extension. We can therefore wrap all existing forms
in formfill statements that include the 'with' control to include
form_errors:

    {% formfill form_fill with form_errors %}
        <form>...</form>
    {% endformfill %}

Because of that we do not have to preprocess error messages and
can remove that legacy code.
Add Nathan Hilbert as a contributor
Patch version 0.16.1

* Better language file handling for all operating systems
* Use FormEncode-Jinja2 to handle errors instead of lxml