Refactor WebException and clean up usages

We should refactor org.nuxeo.ecm.webengine.WebException wrapping in general and provide guidelines to use it (although custom error handlers is already provided).

Exception wrapping should be reviewed - WebException constructors do not do the same process (for custom http error, basic nuxeo exception checking is not done) -> should be reviewed

Also the pattern of constantly doing exception management by hand should be factored into the framework. We don't want each method to have:

    @GET
    public Object doGet() {
        try {
            ...
        } catch (ClientException e) {
            throw WebException.wrap("Failed to do something", e);
        }
    }