🚸 Throw a `ValidationError` instead of `ValueError` when trying to re-assigning synonyms

[falexwolf]

Source: https://docs.lamin.ai/scrna2

It'd be nice to not see a traceback here:

[falexwolf]

And it'd be great if the error told to which record I cannot assign. It's raised in a loop so I have no idea which record it is.