NEXUS-6045: Made content plugin depend on static-resources

As the latter got the duty of rendering error pages (moved out
from content plugin to avoid duplication), plus some other
handy utilities are now reused in content plugin to
lessen code duplication.

Now the content plugin handles only the content pages
and describe pages, while static resources got a new
duty of handling error page rendering, to have those consistent
across whole application.

The only remnant, "real" original copy of the error page
remains now in restlet1x plugin used by NexusStatusService
that was handling error pages while it was mounted at
webapp root.