Currently, this wikitext will error:
<ref group="foo">Note</ref> == Notes == <references />
It will print this mouthful,
Cite error: <ref> tags exist for a group named "foo", but no corresponding <references group="foo"/> tag was found, or a closing </ref> is missing
It does not print a references list.
Render the error at the top of each group, but also render all references. This is for readers' benefit, who will get more from a gracefully degraded rather than a disabled experience.
Side note: we can drop the second part about "closing </ref>" now that the nested <ref> regex also matches an opening <references>.