I don't believe this will actually fix the reported problem, which I
cannot reproduce, but it should give us much better error messages, at
which point we can fix it.
New keep roles/enrolments and groups/groupings options, notification CSS fixed, standardised notification texts, added missing delete of module blocks, fixed file deleting and some other minor issues.
1. We need this so that, for example, when previewing a question from
the quiz editing page, the preview uses the filter settings, theme and
language set for that quiz and course.
Unfortunately, they are used all over the import/export code, so I cannot eliminate them completely. However, I was able to move them out of the core library.