fix(viewer): clean up errors for hidden fields

[pinussilvestrus]

Closes #825

I actually wonder whether this worked beforehand, as conditions never refer to form field IDs, but their respective key.

Seems like we had a lack of test coverage over here. Feel free to check @Skaiir!

[Skaiir]

My changes include:

Filtering of errors is done in the validation method directly. The error state is changed within it, it doesn't make sense to me that the error state should be any different (old bug, I've extended the test case to handle this too).
I've also made conditionChecker default to using a path, but also allowing it to have a specific path function. That way when we want to filter data on a basis other than the object's value path (such as id), we can specify the path function externally.

[pinussilvestrus]

Thanks for the changes! Only one potential test case to add, feel free to merge as you like 👍