Ease CardinalityViolationError debugging

[Famlam]

Upon encountering a duplicate entry, frontend crashes with a non-informative message:
"asyncpg.exceptions.CardinalityViolationError: ON CONFLICT DO UPDATE command cannot affect row a second time"
For "normal" mode it's already pretty time-consuming to debug, but for diff mode it's even much more difficult.

Would it be possible to, every time a CardinalityViolationError is encountered, frontend automatically prints a log statement
CardinalityViolationError encountered. Duplicate entry - item: {...} - class: {...} - subclass: {...} - elements: [{...}]
(in case a duplicate error is the cause of course)

Possibly with extra output like diff_mode: {yes|no} and Encountered: {number of times this duplicate exists}, but these are only 'nice to have'.

I suspect it's most efficient to do this on frontend, as there it could be in a try/catch block and only run in case of an actual failure. Naturally it should still make sure the analyser reports a fail, otherwise it won't be detected.