fix(404): avoid wrong redirection for 404 on resources (#719)

* when the 404 in on a resource (css, js...) and not a page the
redirection to the 404 page should not be attempted. As a result, the
method sendError should only be used for pages. Otherwise we should use
setStatus(404)

* remove duplicate 404 handling

* avoid issue when calling sendError and the response has already been
committed

Covers [RUNTIME-355](https://bonitasoft.atlassian.net/browse/RUNTIME-355)