Improve the UI for not-existing and deleted pages

We noticed users being confused by the similarities between the display for a not-existing page and the display for a deleted page:

• The parent node is selected in the navigation tree in both cases (this was the most confusing as they reported it as "bug": deleted pages still appear in navigation menu)
• The top "Notice" is the same, the only difference is the section "The following versions are in the recycle bin:" for previously deleted pages.

We could improve the messages and make the section "The following versions are in the recycle bin:" more visible.