Missing skin... empty page

I've been upgrading my XWiki from 4.3.1 to 8.0-m2.
And, although the log showed Solr finding pages and pages content, the pages were all delivered empty.
Having dug a little, I realised that the previously defined skin (in database) was colibri and that this was not there. Adding ?skin=colibri to the URLs suddenly made pages (somewhat) appear.

The debug message saying that the skin is not a resource (or something such), a DEBUG message, should be turned into a WARN.