Missing error handling of missing pages and templates for the liveTable Live Data source

livedata-livetable should depend on xwiki-platform-livetable-ui
Otherwise, when calling the `/entries` rest endpoint with the default configuration, the XWiki.LiveTableResults document is not found.

This dependency could be made optional to let admin uninstall if its not needed in practice.

Also, we should check for the presence of the document before rendering it (same for template) and return a more relevant error message when its missing.