Allow to ignore 404 errors

[ogizanagi]

The build command currently allows to ignore specific content not found errors when the content is handled by Stenope (when a ContentNotFoundException is thrown (using the --ignore-content-not-found), e.g: when using the controller argument resolver.

But in some cases, it can be useful to ignore as well 404 HTTP errors triggered by the framework.

In most cases, such errors are problematic and should be highlighted or interrupt the build, because it means a 404 page is referenced somewhere.
But there can also be some case where a GET route with no param is simply collected and generates a 404 in the context of a specific build, while not being referenced in any page (for instance if the link is rendered conditionally, avoiding to render it if the targeted resource is not available).

I had such a use-case where my static build includes a catalog.pdf GET route to expose & dump a PDF in the static build.
But if the PDF file is not available, the link is not rendered in any page. Still, the GET route has no parameter, so it's collected during the scanning phase of the build command.