Improve exception and HTTP error management

Currently the exception management and missing resources are not satisfactory.
In particular:
1. if an exception happen during rendering a template, an exception stack trace is put inside the result, with an HTTP 200 code;
2. if a resource is missing, but it is intercepted by the VelocityViewServlet, it renders with a page with HTTP 200.

It should be:
1. HTTP 500 with error page. If it is not possible (if the response is committed), the error page should be included.
2. HTTP 404 with error page.
In both cases the configuration of the webapp should be considered.